It took forever to start, but did. I had to stay on the gas, even when stopped?

The Engine Computer is by far the suspect. You've introduced a number of new variables when you disconnected the old computer, (or the battery). Along with the original problem you will now have a reluctance to run due to idle speed too low. When trying to start the engine after the first problem is fixed you will likely have to hold the gas pedal down 1/4" to keep it running. You also won't get the nice "idle flare-up" to 1500 rpm at start-up.

Given the symptoms the first thing I'd look at is the ignition switch for signs of melted wires or overheated contacts. The next thing is to check the operation of the automatic shutdown (ASD) relay. You can do that at the ignition coil, any injector, or either small wire on the back of the alternator. All of them will have the same color wire, usually dark green with an orange stripe. This can address the no-start condition but it won't affect the fuel gauge. You should see 12 volts on the ASD wire for only one second when you turn on the ignition switch. That shows the circuit is okay and the Engine Computer has control of it. You might have to do that with a test light instead of a digital voltmeter. Most voltmeters don't respond fast enough to show that first voltage pulse. What's important is that voltage must come back during cranking. If it does not, suspect the crankshaft position sensor.

If all you're going to do with the scanner is check for diagnostic fault codes, you can do that by cycling the ignition switch from "off" to "run" three times within five seconds, then reading the codes in the odometer display. Chrysler makes reading codes much easier than anyone else. You can also use the scanner to view live data. Under the "sensors" menu you can view the crankshaft and camshaft position sensors listed as "no" or "present" but you're doing the same thing by measuring the voltage at the coil and injectors during cranking.

Be aware too that if you're going to keep throwing random parts at it, the crankshaft position sensor's air gap is critical. A new one will come with either a paper spacer glued to the end or a thin plastic rib molded on the end. Those set the gap to prevent breaking the sensor from hitting the flywheel, and intermittent stalling from a gap that's too big. If you reinstall a sensor with the plastic rib you must cut the remaining part of the rib off and use a new paper spacer.