I don't think this is an ME issue...

Aside from the M.E. causing the CPU handshake to be rejected... and making the startup process hang as though theres a bad GPU -- can any other issue cause this?

I've at minimum diagnosed about 8,000 computers in the last 5 years (I own a store)... but, this one had a corrupt EFI (long story, we were using it to test another unit to verify if the EFI was the issue) and subsequently made a PERFECT unit, top of the line 2015 MBPr 2.8GHz with the AMD GPU, 1TB SSUBX and 13 cycles on the battery....... to stop effing working! lol.

Yes, I could have found other solutions, but I wanted to get to the bottom of it - and, due to messing around with EFIs in general in post L13 models, I had a few units that needed their M.E. unf-cked.

Got a device that cleans the ME, fixed all of them but this one -- which at first, didn't fix it, then, STARTED WORKING for like a week for the first time in a year.... and now, is back to literally booting ALLLLLL the way up until the white status indicator loads ALL the way, only to not boot.

Yeah yeah, I think I could take it to apple and snow them in to fixing it. But I don't want to. I want to have a solution for this for the future. Anyone willing or able to make suggestions?

I MIGHT -- get a known-good EFI and replace this ones... but when I edit the EFI to retain the SN -- can I just copy a particular region? How do I know where it starts and stops? I want it to reflect the correct parts for the LCDs QR code, the LB# and the SSD#. (Yes, even if it's NOT under AC).