SonicVQ

Excellent troubleshooting! What an unusual issue.

UpRev or EcuTek?
What is your ECUID?
Did the replacement ECU have the stock programming?

I have seen that in some cases when the ECU is very busy (like uploading a tune) it can create false CAN bus faults. This is because the ECU doesn't have the time to send the normal "heart beats" to the various modules. These are resolved once the ECU is back to normal operation and the vehicle is driven a few times (drive cycles) BUT you can't do that...

If the starter relay is being activated, it isn't a NATS issue...
I assume the "red key" dash light is not on. (verifing it isn't a NATS issue)

I have seen a few cases where the starter wire connection is corroded (between copper lug and copper washers) and it arcs when starting, creating a poor electrical connection. 80 grit sandpaper fixes it.

I know you had the starter tested, but have you tested for 12v at the thin starter cable when someone pushes the start button and do you have a constant 12 volts at the thick starter cable?

Also, look into the B26E8 / clutch interlock switch - that might stop the car from starting if it thinks the clutch is not pressed.
I think you can use a wire and jump the 2 connections in the clutch switch connector to ensure the BCM thinks the clutch is pressed.

Good luck!