I've been working on an 'owner's manual' of sorts wiki page: https://mac68k.info/wiki/display/mac68k/PS2+Adapter

It has discussion on the power situation.

The mac's 5V is not connected to the adapter board. Only 3 of the 4 keyboard wires are connected.

I'd prefer to keep it simple, maybe with small incremental changes, unless someone else wants to do the layout work.