I have a background in electronics - I'm not really up with the latest semiconductors, but I'm OK with good old-fashioned relays! I checked both the loop resistance through the estop switches (measured as expected) and also tried the "short the estop terminals" trick. No go. Also checked that the momentary reset switch was supplying 24V measuring across the relay terminals, and that looked OK. No unexplained voltage drop indicating high resistance on the supply side. Came back half-hour later and all was working.

Then the actual contacts on the "output" side failed, definitively proved by swapping to the unused but identical contact set. That was switching a 24V relay coil used to switch power to the driver PSU, so should be enough contact to keep them clean. I had heard about the need to switch more than trivial currents before, one reason why 20mA current loop was popular, i was told, The internal interconnect contacts all switch internal relay coils, so I would expect them to carry enough contact, aided by a bit of back-EMF from the relay they are switching. Anyway, new relay from Rapid should be here tomorrow, so all my problems will be over...

Except that this afternoon, the machine refused to go all the way through its homing routine. Stopped either while homing Z, or when homing X+A. No idea what that is and had to leave it there, but I'm about to go out to the workshop and either fix it, or bang my head on the wall. One or the other...