Almost cranks but will not quite crank?

That still sounds fuel related, with how it has to turn over smooth it isn't internal engine or ignition related. Neutral switch would shut off the starter. Try this, get a can of starter fluid, now have someone try to start it while you spray some into the air intake, remove the air filter for access. If you can get it to start and run that way then you need to look at the pump and it's connections, then use a pressure gauge to verify the fuel pressure is constant while cranking and that it holds pressure as it tries to run. The other item on these is the EGR valve. They can get blocked open and that causes a large problem when they try to start. You can remove it with the 2 bolts then look into the pintle area. With it off it should be fully closed. GM had an issue in the early 4.3s where carbon in the engine would build up and a chunk could get caught in the pintle and the engine would stall out and run like poo if you could get it started. The solution is to use a cable on a drill and run it into the passages on the intake to knock the carbon free and then start the engine to blow the stuff out. There are Clean-Screen gaskets that can help stop the problem if it is carbon. My 94 had it happen to it. Hearing how yours was trying to start caused flashbacks.

You should be able to see if there is any carbon in the EGR as it would hold the pintle off the seat and there would be a gap around it. Shine a light into the rect. Port and see if you see light. If no light bolt it back on, but plug the vacuum line to it and then see if it changes it at all. There is a control solenoid that turns off the vacuum to it, if it went bad it could hold the EGR open. Bolting it back on disconnected would block the passage, if it now starts and runs you would look at the solenoid. If there is no change then we need to check something else. Like actual fuel pressure.
The issue isn't the distributor. It wouldn't cause this issue and instead it would either not run at all or it would be backfiring through the intake or exhaust.

If it still acted the way it was in the video even with the spray it may be an internal issue. However I would still check the timing and compression. It's not common but it might have jumped time at the timing chain. That usually creates odd symptoms though, not just a hard and long crank like you have but backfires and pops which you don't have. It's also possible the PCM is acting up but unless you have an OBD I capable scan tool that is hard to check.

Timing light on cylinder one and aimed at the balancer is the only reliable way. You can generally get one as a loaner tool if you don't have one.

Sorry there isn't, not to test the actual timing.

From the description your problem is likely a failing ECM. That would get you odd behaviors and now the lack of code reading. You might want to hunt up a scan tool that can do OBDI The Snap-On MT2500 is a good one for that. That would tell you much more than trying to read flash codes.

If you are using the same sensor there is no relearn, that is used when you change sensors.

Flash 12 just means it hasn't stored a code. If it was mine, I'd start at the basics. You know it turns over, so that's okay, you had it running before so it should have compression and be in time. So that leaves spark or fuel as the likely culprits. With the way it has progressed as it is getting worse, I would look at the PCM itself as being the item that is failing. It could be as simple as a bad capacitor in the PCM that shuts off fuel or ignition as soon as it heats up. I would probably proceed by using an OBD I scan tool like a SO MT2500 or OTC Genisys to watch the live data from the sensors that it can show and see if there is something there that gives a clue.

That would be a self-tapping bolt. If you were to look at a new one, it would be tapered and have a triangular profile at the tip. Tons of them used on vehicles.