Well I'm ready to be done with this adventure.
I've poured all the blood sweat tears and resources into this POS that I care to. It's still f*cked and there doesn't seem to be any way the un-f*ck it.
I got my transmission external wiring harness adapter (for $117) yesterday and very carefully went over every wire before I installed it.
Dropped the pan and replaced the internal harness and got all of that squared away.
There is no damage to the original harness that I can find, and the result is the same, MIL and limp mode.
I'm so mad I can barely type this out.
The tech info I've been able to get is horrid and the build quality is worse.
At this point I either put a haltech on this thing or burn it, but GM can kiss my ass.