I built the first prototype and found a couple of errors on it:

1. The quad NAND gate has the wrong pinout.
2. There was no connection from the Pi GPIO header for the GFI test line

Both of those have been corrected and in addition, I replaced 3 diode-plus-pull-up level shifters with a triple bus buffer chip. It may be slightly pricier, but placing a chip and a bypass cap beats 6 passives. I also replaced the dual JK flip-flop with a single D flip-flop chip. The fact that it's a D vs JK doesn't matter in our case because we only use the set/reset functionality as a latch.

I was able to test the pilot generator and ADC and they work properly. Also the relay test input works (though there wasn't much chance it wouldn't).

On to v1.1!