I thought ecu's were one time use and locked but as you say you maybe able to overcome this by your method with truecode REE1 software.

But I would think you may need to use a new key fob as this I'd guess also be locked to the uch & ecu.

The problem I can see is how will you get the code to match the used ecu because I thought you need to know the truecode or immobiliser code to learn the uch and pair it with ecu.

Wouldn't it be easier to get a set from a donor car.