VY BCM Heatbeat issue

[gmfCars]

Hi all,
My old man has a 2004 VY V6, the engine needed to be replaced, so he replaced it with one from a VX which had a stage 2 cam from MACE Engineering, he hadn't realised at the time but the calibration couldn't easily be swapped over, the car was running with VY ECU, but stalling and failing quite a bit, going against the advice I had given him he installed VX ECU onto the VY Harness (by removing the plastic on the connectors), now the we have a bunch of electrical issues.
The first major note is that there is no BCM Heartbeat, nor any chatter on the communication bus, along with that there is the following which may be or may not be corralated or causated by the above:
Starter Motor not working (power delivery issue, probably due to ECU/BCM right?)
Coolant temperature sensor uninstalled,
Fob not working,
Drivers door requires different key (relevant?)
Engine Cont relay, very hot when in ignition.
Alternator high pitched noise
Fuses/Relay all seem to be in order

Equipment and resources:
Laptop, Multimeter, Envyous customs ALDL cable, IT/Coding Knowledge, basic electronics knowledge, passion for complicated scenarios,
Various tuning/diagnostic softwares.
Would it be possible for me to track down the issue and fix it?
My first and initial guess would be that the BCM or another electronic module has been fried because the pinout of the VX ECU/PCM is different from the VY ECU/PCM (correct?)
What would be the best approach for troubleshooting this?

 

[Fu Manchu]

Go back to the VY ECM matched to the car or the car will be a driveway decoration.

 

[Fu Manchu]

Pinouts are very different.

 

[Pollushon]

This might help as some VX were VY compatible but it's going to be much easier just getting a tune on the VY to suit

https://pcmhacking.net/forums/viewtopic.php?f=7&t=152&p=4323&hilit=Flash+PCM+wiring#p4323

 

[gmfCars]

Go back to the VY ECM matched to the car or the car will be a driveway decoration.

The VY Ecu has been put back in it, but the issue I mentioned still persist.

 

[gmfCars]

This might help as some VX were VY compatible but it's going to be much easier just getting a tune on the VY to suit

https://pcmhacking.net/forums/viewtopic.php?f=7&t=152&p=4323&hilit=Flash+PCM+wiring#p4323

With the info you provided I should be able to test the voltage difference between those pins using my multimeter, would doing that be helpful?
Also MACE isn't really willing to give my dad any tuning files for free and they want him to buy an ECU directly from them, which is fair enough, So question: would it be possible to find a "calibration file" online or calculate the values for the calibration by hand to then upload the ECU once we fix the electrical issues?