If all you can get is 16mm² cable, then put two of them in parallel.
2x16mm² would have a lot higher current carrying capacity than 1x28mm² cable.
The other thing I would check would be that the new cables had similar thickness strands of copper, so that they have similar flexibility
i'm really surprised! you have really a good knowledge in diagnostic techniques,
any way upon to my experience that car has faulty ecm motherboard , you have to make sure in case if you disconnect the ecm you will loss " ROM" the keep alive memory or not because the same thing with camaro , if not you can disconnect ecm and see voltage will drop to zero at ...