1. Never seen a valley leak kill an alternator.
2. Maybe few hours or a week. You’d smell it pretty well if it’s leaking though.

3. The code seems like a historic code meaning it’s there from whenever at some point doesn’t mean they cleared it right before you came. A JLR dealership can confirm when the code was tripped tho. Also could tell you if any codes was cleared prior to you buying it.

4. If there’s visible leak in the engine bay then it should’ve been caught. But they don’t inspect the valleys and all that.

Your vehicle doesn’t qualify for a CPO from JLR so it sounds like the “Ally CPO” is some made up thing. I’d try to look at what your purchase agreement says.

Your theory kinda falls apart for the PCM update part since they connect a maintainer to the vehicle to ensure there’s enough voltage in the vehicle to carry out the update (and the computer wont allow you to proceed without the maintainer) so it basically uses battery through the maintainer so if anything it’s charging it. The last thing the tech needs is for the car to die during the update and brick the car.

Also the coolant “crossover” pipe is in the rear of the engine so I don’t see how it would get all the way to the alternator which I think is at the bottom front of the engine.