Boy you've been busy on this one. With all of the things you've done, checking the usual suspects, it kinda sounds like a fuel injector with a runny nose, as you've noticed it acts like an over rich fuel condition on a restart & the psi leaks down in an hour. When you replaced the plugs, did you do a cyl plug read as you removed them???? Do you remember Any particular cyl spark plug looking more carbon fouled than the others???? If so, maybe pull that cyl fuel injector for inspection & or bench testing.
Any trouble codes set, or pending???? Did the problem come about suddenly after some event, or slowly over time????
More thoughts for consideration, let us know what you find.