Just read it from the beginning to catch up.
First off,
@TylerHagerdorn, you gotta chill a bit. I understand this is a very frustrating situation. We've all been there. I also understand how frustrating it is to have to repeat yourself. There's a lot of activity on this forum and we can't possibly remember everyone's latest update on their project nor are we much inclined to research every post or thread a member made to collect the info we'd need to try to help.
Cool off and be patient with us just as we're being patient with you. Help us help you.
From what I gather in this thread, I'm banking on something flooding the engine. That's not to say actually filling the cylinders with fuel. Holding the pedal to the floor (or at least beyond a certain amount, like 80% or so) when cranking is a factory function called "Clear Flood Mode". It stops the injectors from firing and I think it kills spark as well. The system losing pressure absolutely indicates a leak. If it's not leaking externally, then it has to be leaking back into the tank or into the engine. Based on the hard start problem, I'd say it's leaking into the engine. The injectors are what controls what goes into the engine so I'd be suspecting leaking injector(s).
My first step would be to see what all the sensors are telling the engine via a scanner that shows live data. This was OBD1, but I had a '91 Blazer that kept running rich and fouling O2 sensors. It ran so rich that it clogged the O2 sensor enough that the sensor couldn't "smell" how rich the exhaust actually was, so it wouldn't throw the "rich exhaust" code any more. After it clogged the cat, I finally had the sense to plug in a scanner and found that the ECM was constantly trying to warm up the engine because it saw that the coolant temperature was WAY below zero (summer in south Louisiana). Replacing the coolant temp sensor fixed everything. I think that 4.3 had two temp sensors- one for the ECM and one for the gauge. The LS has one sensor, so, your gauge would be reading low if it was a sensor issue. Unless, like many GMT800 clusters, your temp gauge is broken or otherwise inaccurate and you haven't mentioned it. Even still, adding extra fuel for warming up happens after you start it. So, I don't think it's a coolant temp sensor fault, but there are other sensors and it'd be a quick and easy starting point.
I don't know if you can simply hang the rail with the injectors stuck into it. It seems to me that the fuel pressure could push the injectors out. Maybe zip-tie or wire tie them to the rail? I'm picturing hanging the rail as high as the hose and injector harness will allow. Maybe lay some 1x2s or similar across the intake manifold and tie the rails to it? Maybe slip cardboard or long trays (like drywall mud trays) underneath the injectors. Be sure to unplug the coil packs.