possible severe engine damage - would appreciate diagnostic advice

[mikefocke]

Assuming there is some internal damage, it will only get worse. Any revs will only spread any debris throughout the oil passages and can lead to the need for an expensive engine rebuild or replacement.

I'd immediately read any codes for clues. Post the codes.

Pull the oil filter off (without starting the car) and examine the spread out filter element for any signs of debris. Post pictures if you find any and describe the color and composition of the debris.

Pull the serpentine belt and feel if there is any play on the water pump.

And, if you don't feel able to do those things, I'd arrange for the car to be flatbedded to the most knowledgeable mechanic specializing in the internals of the M96 engine that you can find. Without starting it.

My reasoning is that spending time and money diagnosing reduces the risk of having to spend even more.

Good luck...I'll hope for the best results for you.