I decided to start a new thread since everyone agrees my Yukon is very sick.
The dealership and my repair guys agree with the diagnosis of inverter and transmission.
Some of what I'm writing is based on information from the internet, Youtube, and other sources.
If I'm wrong please correct me so I can adjust my analysis.
My IT and electronics experience, along with a small amount of optimism, not sure where that came from, is thinking that you can't accurately diagnose a transmission problem without having first solved the power problem.
If the inverter has failed, ie: not consuming DC current from the HV battery and producing three phase AC current, then you can't say the drive motors have also failed.
If a horse is standing on your foot causing pain, you can't determine if your foot is damaged until you remove the horse.
I'm thinking if I replace the inverter / drivetrain control module, then we can determine the extent, if any, of damage to the drive motors.
GM says the drive motors aren't replaceable and only available as part of the whole transmission assembly.
I watched a YouTube video of them disassembling this transmission, not what I had envisioned about electric motors driving it.
GM wants $3k+ for a new inverter, I'm hoping I can find a used one for less, regardless after replacing the inverter I'm still a long way from the $16k GMC wanted for replacing both items.

I'm not sure what I'm asking for, kind of typing and rambling at the same time.
Do you guys/gals have any thoughts or insights into my idea?

This Yukon was our first adventure into the world of hybrids.
At this time the used car market is insane and people are paying incredible premiums for new SUVs.
I can't justify dropping $16k in repairs on a truck with a used value of $6-10k.
I really can't justify spending $60k+ on a new Yukon or similar.

Okay I'm done rambling.
Please discuss amongst yourselves and let me know if I'm crazy or just a little bit off.
I'm in central Florida where it's now summer so I'm not crazy from the heat but not far from it either.
Thank you for your time.

 

Your thinking is reasonable. I would add that you would want to check the transmission anyway to rule out a bad transmission damaging the inverter module. What makes GMC think the inverter is bad? Could it just be detecting a failed motor or phase cable shorts? maybe do some checks on the motor phases to make sure the resistance measurements match between phases. check between each phase and the shields. a short in the motors could possibly damage the inverter, but I would hope GM programmed a fail-safe into the inverter that would detect over-current and open the circuit to protect itself.

One failure that has been seen before is damaged motor phase cables between the transmission and the pinch welds nearby. the phase cables shield gets damaged. Your codes sound like the actual phase voltage is shorting out rather than the shield, so maybe the damage is more extensive? Check the 6 orange cables going from the inverter to the transmission.
Take a look at this thread about the pinch weld and the motor cables.

Looking into your codes a bit more, it appears the problem may be in the HV cables between the hybrid battery and the PIM (power inverter control module):
Codes P1AF0 (Drive Motor 1 Control Module Hybrid Battery Voltage System Isolation Lost) and P1AF2 (Drive Motor 2 Control Module Hybrid Battery Voltage System Isolation Lost): The drive motor generator power inverter control module(PIM) contains two motor control modules(MCMs). Each MCM measures hybrid battery high voltage(HV) with several internal sensors. The MCMs test for loss of isolation(LOI) between either the HV positive circuit or HV negative circuit and vehicle chassis.
Did the GMC dealer or your local tech actually check out the HV battery cables? They could be shorting to chassis.

 

I doubt GMC or my guys examined the cables for damage.
This whole failure started when my guys went to start it after an oil change and replacing an ABS sensor on a rear wheel.
They said it made a loud whine like a jet then a slowing spin down sound.
My guess is the inverter sent voltage to the drive motor #2 which caused the motor spin up and down noise.
I'm also guessing those two AC motors probably spin with some serious torque and rpms so any surge the inverter can create wouldn't be more than the motor could accept.

My repair shop doesn't have much experience with our hybrids but he's done considerable services on the Prius and other popular cars.
I've printed this thread and the other one you linked regarding inspection of the cables.
I'm going to let him read through this and get his opinion using this new information.
I would love to get our Yukon running again for only $4k (only, can't believe I said that) but if he's not convinced then I will give up and put it up for sale for parts.

Thanks for your help.