Update: I'm calling this issue resolved and returning the truck back to service. Finally!
After sitting 24 hours it will now fire up after 1-3 seconds of cranking. I haven't left it sitting for many days yet, hopefully that will be fine too.
In the hopes it can help somebody else, I will summarize here the things I found that were actually causing the problem, in addition to some other improvements along the way.
The top 3 that finally resolved the issue:
1. Re-routed injector return lines to exit from front driver-side injector near IP and tee into the line from IP which now runs along the side of the engine, so the whole line is easy to inspect and repair. I had first diagnosed leaky return line by cutting the line under the truck, plugging it, and then blowing compressed air into the hose where it connects to IP. Lots of hissing at inaccessible area at the back of the engine was a real big clue.
2. Replaced faulty check-valve at top of IP. This was allowing air from (leaky) return line to enter IP. Once I did this, it went from "no fire at all" to "fire, then die". So, an improvement. After replacing the check valve, I blew compressed air through the old one and was surprised to see the little ball snap back into place and now the valve looks exactly like the new one and I can't blow through it anymore, so I probably could have saved a few bucks.
3. Replaced faulty check valve between fuel filter housing and #1 injector (return line). I could easily blow through the old one. This enables fuel filter air bleed mechanism to work correctly without air getting back into the filter.
Things that helped along the way:
1. Developing a clear understanding of how fuel flows through the system, and in particular the two check valves and return lines.
2. Installed electric fuel pump and removed and blocked off the mechanical pump.
3. Installed a napa fuel filter with its own drain and deleted the factory water separator.
4. replaced the injectors with some remans from russrepair.
5. replaced o-rings, caps, and return lines between injectors.
6. removed and plugged the fuel pressure sensor. ( I might put it back soon )
edit: Well, I was going to link to this post from the thread-starter and also mark the thread title "resolved" so anyone starting reading could find the solution, but the forum software won't seem to allow me to make any edits, I guess because it is an old post. so.... bummer.