That is the crazy part, isn't it?
I'll go through the wire tracing with the cluster out per your writeup, but I've checked continuity on the connections end to end on the circuit board before and it all seemed OK. I will be surprised if I find anything wrong there - but you never know!
Replacing the nuts on the gauges might help but other than that I'm really out of ideas. Seems unlikely it is both gauges failed at the same time, and both seem to work when the sender wire is grounded. And, the IVR is pulsing the voltage like it should per checking the sender end of the wire.
I don't know if I did or not, but there are other things grounded to the dash that work so I think it's good to go there.