15" MacBook Pro early-2013 retina: no device will boot this machine.

I’ll try to be as concise as possible:

I have a 15” early 2013 MBP that I gave up on a while back because it would not boot; it would hang halfway through the progress bar. I was and still am not able to boot from a USB installer, Internet Recovery, a target-disk other MBP, etc… I can see the hard drive on my other MBP if I boot the faulty unit into Target Disk. I can format it, I was even able to install the OS to it over thunderbolt. But no matter what, it sits and gets stuck at the progress bar, either in the middle, or just towards the end. When booting from internet recovery, only after a fresh format from target disk mode, it will get to the point where it wants to load (older version, spinning wheel) and then freeze right there.

I checked the time using single user mode and modified it to be current, but once the drive is wiped with no OS single user mode doesn’t work.

Reset the PRAM, SMC, etc…

I even pulled the unit apart a while back and gave the logic board a once-over, everything looked fine. I tried this whole process with the SSD removed also, and I get the same problem.

Verbose mode left me hung with a weird command that said “setting btcoex config” and that’s it.

I’m at my wits end with this machine and want to think it’s the main logic board but I don’t want to shell out money and find it wasn’t the issue at all, plus I know there is technically no way to flash the firmware on the logic board without an OS installed so I’m in a bit of a jam with this. Anyone advanced able to offer some assistance?

FYI I have (until recenty) been a certified Apple tech since 2003 and I do this stuff every day, but this one is perplexing me.

Even crazier - I can install Windows on it. I just checked. Not Mac OS. Windows...