Check Engine light on since 2013?

I completely agree with you 100% but this system is broken, and nothing is easy. Believe me, I understand what you're saying. I'm working on a truck right now that is supposed to bring supplies to people in Ashville next week, and there isn't anyone else to work on it except the dealership and they would end up having it for a month. Talk about feeling pressure.
You can try to de-pin each wire, but you will find that it is difficult, and most of the time if you have never done that type of repair the pins and wires end up getting damaged. Plus, with a new pigtail and connector, you can be sure that the pins inside the connector are tight and not spread apart, which often happens when connectors get into that condition.
To splice in a new connector you only need to go wire by wire, just do one at a time, stripe off about 1/2in of insulation of each wire, and crimp them together tight, you shouldn't be able to pull them apart easy, that's all you need to do, then you just heat the connectors and you will see the heat shrink cover the wires. Honestly I'm not trying to make things more difficult for you. I would never do that to anyone. This way once you plug that connector in, it shouldn't be something you need to unplug again.

Watch this video, it's just a 3min or so quick video that shows how fast you can do this.
Wire repair video....
https://www.youtube.com/watch?v=sNdT23FYl-A

This is a guide for checking for spark when cranking the engine over, but I you won't have spark without the distributor connector in place, that's where the pick-up coils are for the crank and cam sensors, if those signals are missing, you will not have spark. Have you taken any of the spark plugs out to check their condition?
https://www.2carpros.com/articles/how-to-check-for-ignition-spark

This is a guide for checking fuel pressure:
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

And regardless of the distributor, you still have low reference voltage, that is going to affect starting as well. Thats where you began with, codes for sensors that showed to have low 5volt references. That's why those codes were setting. Take care of one problem at a time, with a malfunctioning ECM nothing is going to work. I'm looking back at all your testing results right now, there is low reference voltage to multiple sensors, which will not allow the engine to start.

I dont think it has anything to do with the AC compressor, it has a clutch on the front of it and the clutch needs to be energized for the belt to drive the compressor, You would hear the belt screeching away loud because the belt would be slipping on the compressor pully. Check for ignition spark first. When diagnosing a crank no start condition, always start with the basics, Spark, Fuel, and Compression. Checking for spark is the easiest so start there. Heres the guide for ignition spark. This issue has nothing to do with running the AC compressor to check the position of the idle speed motor.
The VIN you gave for the used ECM does not come up as any vehicle at all. Recheck that.
If youre that concerned about the AC compressor and it looks like it runs on its own belt, meaning nothing else is driven by that belt, just cut it off. That will take the compressor out of the equation all together.
The best way to check for spark is with an adjustable spark tester, where you can adjust the gap to stress the coil somewhat, but use what you have.

https://www.2carpros.com/articles/how-to-check-for-ignition-spark