7C81 - It appears that I have the wrong BOB, what now?

That's disastrous news!
But I suppose it's a watertight answer for my problems...

I don't suppose you have any insight as to when the 7C8x will be back in stock, would you?

We will have the 7C81T later this year
plus we may have a few boards with solder issues
that can be fixed

It's likely your 7C81 is repairable
Most like U1 is damaged
(Usually from high negative input currents damaging the input clamp diodes)

You can check by measuring the voltage across C14, it should be ~4.2V
Another option is to use P2 instead

I'm measuring:
24.7mV across C14
4.12V across C15
4.16V across C32

I removed the Pi and checked all GPIO on P2 and P7. They read 5V across the board.

That is evidence that the 74CBT16211 (U1) has been damaged.

I like the sound of 7C81T,
are you keeping the RPI header the same as the original?

PCW wrote:

Another option is to use P2 instead

I tried to do exactly that - but instead of continuing to struggle, I thought I'd ask for a hint for how I'd do this. (I think it's reasonable to assume I'm doing it wrong.)