I had a similar issue in a 5.0 chevy in my old T/A..
It just up and died one day.. no spark etc.. all the voltages to the throttle body injectors were perfect, power to the ECM was fine and I just couldn't find a reason for it to not start up after a day of testing voltages parts etc..
All the signs were there for it to start and run just fine until I actually turned the motor over, then nothing would happen.. test everything.. rechecked parts etc.. nothin..
I gave up after two days of testing everything I could and had it towed to a dealer for them to have a go at it.
The service guys were at a loss as everything they did showed that it should have been running perfectly, but for them, it would not start at all, not even an attempt to start.
THEN a tech did something pretty much like anyone else that is giving up would do.. threw up his hands then leaned on the car.. it started right up!
He jumped up and it died...
Prognosis - positive battery cable ECM lead.
It had a break in it that showed full voltage as it should while not being used, but as soon as you turned the motor over, it heated just enough, instantly, to **** the power to the ECM, when letting off the starter, it cooled enough to make the connection again and allowed reading full voltage through the cable. When he leaned on the car, his hand touched the cable, moving it just enough to keep the connection, allowing it to start.
Not sure if that will help as tracking down things like that can be a pita going it alone, but may give some food for thought on tracking the problem.