Ok, you did not mention a compressioin test but did check valves/rockers persay. I would consider a compression test to rule out internal/mechanical but really,, I am not leaning that direction
Knowing all you did, tried and tested leads away from simple, obvious items.
Now,, the most common problem with theis engien design is the fule meter body or the "spider injection" as we call it
They are NOT fule injectors but "poppets"
They were notorious for problems
Sure, we try to clean first but that only works say 50% of the time
When everything has been ruled out and leads to fule delivery on a cylinder? 99.8% of the time, you have a bad poppet
You would then purchase a new fuel meter assembly as shown below, remove only the upper intake, change it out
They have been redesigned and improved for all the past problems they had. You can buy at most any local parts store
Now, up till 2003 GM had a recall on the fule meter assembly./ It was for 10 years. 200,000 miles, whichever came first. You are out of that recall so the repair is on you at this point
This is so common. We have changed many of them in the shop
Im am near certain you have a bad poppet
You have ruled out everything except compression test