Where is the vapor canister vent valve located?

I don't know what he means by the ECM is shorting it out. That doesn't make any sense. You checked the resistance of the Purge valve, If the valve was shorted out (low resistance) it would take out the ECM driver that controls the Ground side of the Purge valve. Which may have happened, but it's a pulse width modulated circuit, it's controlled by a 12volt square wave signal, so at 50% duty cycle the valve is powered 50% at 12v and 50% at 0v. This would be seen on a lab scope. But that valve shouldn't be hanging open if it's unplugged, there's no way for the ECM to keep it open if it's unplugged. So, there is either too much pressure on one side and manifold vacuum on the other pulling it open which I've never seen, or something is getting into the valve keeping it open.

But I didn't know you were getting the P0496 first, that can be a Fuel Tank pressure sensor issue, or the sensor is showing the issue. If you check out on you tube some video's on how the purge valve works on Chevrolets it will give you a better idea of how things are controlled. The Tank pressure sensor is an extremely sensitive sensor, it doesn't measure like other pressure sensors, it measures in H20 which is very small amounts of pressure, that way it's very accurate.

But when did the overcharging issue happen and how high did the voltage go? You should be able to tell with your multimeter if the battery has a shorted cell. Just see what the cranking voltage is, if the battery is dropping way down, below around 9volts, then the battery is not holding a full charge. But an overcharging condition can damage the ECM. So, there are a lot of variables here. Excessive current flow from the charging system can do all kinds of damage.

I would ask a dealership if they have seen an issue like this. Tell them about the valve being open with the hose off and unplugged, see what they say. But you have to include the charging system info as well, it would take live scan data to see what's going and if someone can bi-directionally control the valve, which it may not be able to do with that code set. Most systems will intentionally shut down systems that are malfunctioning. Right now, all we're seeing is bad valves or valves going bad. Or some other fault in the evap system. I can only go by what you are seeing happen.

Okay, yeah so, you've got some other weird issues going on too. I'm assuming it fried to 100amp fuse for the charging system? And the 11volts isn't good either. Another concern would be with the old alternator over charging the system, which means that there was higher than normal ac voltage ripple. A healthy alternator has a set of diodes in it that change the ac voltage into dc voltage that the ECM can handle. When alternators go bad, even just the regulator, if it didn't damage the diode trio it's still going to be outputting a higher-than-normal ac voltage. If it did damage the diodes, who knows how much ac voltage was hitting the ECM. At this point I might even measure the ac ripple just to see where it is at. Most people will say it can go up to 100mv but I don't like to see over 50mv. I guess that is beside the point now, but it's hard to tell if any damage was done to the ECM during all this. 11 volts on an actuator circuit is not a good sign.

I'm going through the wiring diagrams right now and noticed a few strange things. Are you getting the 11 volts while it is running? Because I assumed, and that's never a good thing, that the purge valve was getting powered up directly through the ECM, but that's not the case. I'll put the diagram below, it gets its power through the EMS2 EF25 Fuse. That fuse also powers up two oxygen sensor heater circuits, the mass air flow sensor, and the purge valve. If the purge only has 11 volts on it, then the rest of these components may only have 11 volts too.
So really you should be reading almost full battery power on the purge feed wire. You might want to go through the fuses in the driver side under-hood fuse panel and see if you find anything strange going on in there. You can see the other fuses in the diagram that are located in that box.
And if you find 11 volts on that fuse, I would do a voltage drop test while the car is running and turn some lights and accessories on to load the charging system. Put your meter on that fuse and the battery positive. That will tell you the voltage potential or voltage that is being lost from the battery positive to the fuse. Which really should be close to nothing.
And do the same for the ECU EF 14 Fuse 10amp. If there is a difference between those two fuses, then there is too much voltage drop across that Engine Main Relay. If it's 11 volts on the ECU fuse, that is a problem. That feeds directly into the ECM pin 12 X1 connector.
I know we're getting away from the purge valve here a little, but we should make sure that this 11 volts isn't happening everywhere.
Also go through and check the rest of your ground connections. To the engine block, to the chassis, because this ECM only has one ground connection. And it's down near the starter. It's a black wire. I'm surprised the ECM only has one ground, that really can cause so many issues. Is that the one you found loose?

Ahh im glad you tried that, to be honest I dont know why they make parts that can even be put on backwards like that. The manufacturers should know that someone is going to obviously take the valve off and bring it to the parts store, then they get back home and forget which way it goes. Thats not your fault at all. They should have the hoses with specific fittings so that cant happen. And yes I think the stalling was caused by that valve pulling from the fuel tank when it wasnt supposed to. It should only purge when there is fuel vapors. If theres no vapor stored in the canister then its just pulling air in threw the vent valve. Thats pretty much just a vacuum leak at that point. I would try driving it a couple days and see if the ecm shuts the check engine light off. It might take a few drive cycles, key cycles and/or self-tests for the ecm. It takes different temperatures and amounts of gasoline in the tank for certain tests to run. Things should be working correctly now technically speaking.
If the tank is low on gas you might try going and getting some gas and keep driving it. If something is still wrong it might set a different code. The code indicates vacuum during a non-purge condition. So if its a faulty Fuel Tank Pressure sensor, it will set a code for it. As you can see below, theres a bunch of conditions that need to be met for the code to be set.
So try some more gas, just dont over fill it, when the handle at the gas station clicks off, dont try to put more in. Some people do sometimes, and that causes fuel to get into the canister.
Also that Ground you found loose. Since that is corrected, I would also check the ground wire down near the Starter motor. Make sure its good and tight. If you need to take it off to clean it, you might want to take the battery negative off, because the ground at the Start is the only ecm ground.
How is the car running now? With that purge valve being open all the time, the ecm was compensating for the extra air by adding more fuel to the mixture. Increasing the pulse width on the injectors, So a good run on the highway to clear any build up out would be good. Let me know how things go. Im super glad you got that part figured out. It was bothering me, Ive never heard of a valve hanging open for no reason. A learning lesson for us both.
The reason for the p0443 was that when the valve was open, it was essentially completing the circuit, and the 12volts was going right threw the valve back down to the ecm on the control wire, and that wire has a voltage sensing circuit on it. So every time that valve bounced completely open it would be sending voltage to the ecm and the ecm saw that as a short to voltage and set a code. A made a little diagram (2nd) so you can see how it detects the voltage reading. Hopefully the new valve is not damaged. You might want to check its resistance after it gets hot, like after a good drive. Most electronic components malfunction either really hot or really cold. Sorry for the long message.
Just saw the new message, is your battery at 11 volts with the car off?

The battery voltage should definitely not drop below 12.4ish, even with the engine off. If it goes right to 11v when you shut the car off, I'd say that's a cell issue. If it dropped of slowly, it could be more of just a weak battery. But if a cell is shorted out it would drop right away. Each cell is like a small 2v battery. 2x6cells gives you the 12v. But a shorted cell can also damage an ECM. I really don't trust what advanced auto or AutoZone say when it comes to testing things. I have no idea what kind of machine they use, but when it comes to batteries there's no telling if they are load testing them. The p0443 will come back if the sensor is unplugged while it's running, so don't worry about that. It's a circuit code that's why.
I'm not sure of all that Chevrolet does now with code setting criteria, I know Ford for example in some year vehicles will shut the light out or won't show the code anymore for a certain number of key cycles or drive cycles. So that might be why you can't erase them right now. With a more advanced scan tool they might be able to be erased right now. My Autel will erase codes right away, but if there's an issue they will come right back. Sometimes for testing I will need to erase a code to get a circuit to start working again. So, I will have about a 10second window to catch an issue on my scope before a code sets and the system is disabled. I just went through this with a Toyota. 10.6sec to test a circuit driver, then it was intentionally shut down.
For the battery though, it might have an issue due to the overcharging. It doesn't sound right though. Do you have another battery you can try? Sounds like the charging system is ok. But drive for a bit and see how it is. Let's see what codes come back after a couple days. Unless you notice something that you're sure is not right.
Btw thanks for the nice compliment, I really appreciate it. Glad to help anytime.

I was actually just commenting on the converter when you changed your message. Depending on how long the engine was running rich due to the purge valve issue, the extra fuel going down the exhaust could have damaged the converter. The worst thing about it is the rich mixture. It melts the cat down and you would definitely have a lack of power and sluggish response like that. When the honeycomb of the converter starts to melt, it starts blocking off the exhaust and the engine can't breathe correctly. There would be excessive back pressure. If there was a weak spot in the exhaust, even the exhaust manifold gasket, it could eventually push its way out.
The high idle issue might still be something to do with the filler neck. If you're taking it somewhere, have them do a smoke test on the evap system and the intake manifold to see if there's a leak somewhere. I'll bet there is. That will cause strange surging and high RPMs because every time the purge valve opens for normal purging, it's drawing in outside air too when it's not supposed to. It's technically a vacuum leak. I would have them inspect the converter too, check to see if any of it has started to melt down. Unfortunately, you're running into multiple problems at once because of the poor running conditions you started with.
So, make sure they check the evap system for a leak, the catalytic converters condition for meltdown and possibly the oxygen sensors. You don't want to run into problems a week later with more damage. Or the check engine light coming back on. Below I'm putting some "real fixes" from the p0443. Hope it goes okay for you bud.

So, you're saying that the fuel trim is positive at idle (i.e. 5-10+)? Lean at idle points to a vacuum leak, the fuel trims and oxygen sensors should go rich when accelerating. But the lean at idle means the oxygen sensors are reading air in the exhaust at idle, so that's either a vacuum leak into the engine or you mentioned an exhaust leak, they will allow air into the exhaust and fool the oxygen sensors sending them lean, therefore the ECM enriches the mixture. With an exhaust leak the ECM is being tricked by the oxygen sensors. With a vacuum leak the mixture is actually lean and the ECM is enriching the mixture to compensate for the extra air.
The short-term fuel trim is more of a correction of the fuel trim constantly, whereas the long term is more of a learned air/fuel mixture. Yours is compensating so fast because the mixture is changing so rapidly.
The way to test for a cat going bad is to take a back pressure reading in the exhaust before the converter. For example, I would take out the front oxygen sensor and test for positive pressure there. It's a very slim margin. The max back pressure is 1.5PSI, barely anything. If there's more than that, the cat is in trouble. You can also look at the oxygen sensor after the cat. If it is not a stable reading 450-700mv (in that range). If it's swinging like a regular front oxygen sensor from 200-800mv back and forth quickly, that's a sign the converter is not doing its job.
Your long-term can't stable out because of the false air getting into the system, either into the exhaust or leaking into the engine. There's too much variation for it to learn a fuel strategy.

Heeres a couple guides to help. But the car needs to have a bunch of components checked out. You don't want this to get worse. Lean mixtures get the cylinders hot, you can eventually burn valves and things you don't want to see happening.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/engine-surges

https://www.2carpros.com/articles/how-an-oxygen-sensor-works

I would have the shop you're taking to unbolt the front of the cat and have a visual look at it. Thats the best way at this point to see if the front of the honeycomb is starting to melt down. Have them check that before you replace the MAF, because the catalytic converter has been through a lot with everything that's been going on with the car and if it's bad, they can be expensive. That would be my first concern. If it is on its way out and they replace it, you will notice a huge difference in its performance. Once the engine can breathe correctly it will be back up to power. If there's holes in it now, then its already done for. So, concentrate on that and have the evap and intake manifold smoke tested for leaks. After that let's see how it runs. I think you're going to see a big change in how it runs. Don't spend money on extra parts you might not need yet. Save your money for a catalytic converter.

Are you able to accelerate with full power now, or is it still sluggish? I disagree with the hunting idling issue. The flex pipe is pretty far down the line. And he could have checked the converter by taking out the front oxygen sensor and taking a look with a bore scope. I do it all the time, just because a p0420 or p0430 has set doesn't mean the converter is fine, but if he didn't want to check its condition, that's his call. If you can see the data PID for the MAF on your scan tool, at idle it should read the grams per second (g/s) close to the size of the engine. So, for example, my truck is a 4.0l, so at idle it will read around 4g/s. Thats a rule of thumb for checking a MAF, I just hope the car is running well enough for you, having enough power. With all this that you've been through with this car. Before you change out the MAF again, take a look at the throttle plates, make sure they're clean and no carbon builds up around the edges. I used to live in the rust belt, so I know everything has a half-life up there. How are the fuel trims doing now? Check them at idle and at 2,500 RPMs.