So I got the machine physically moved in now. I checked all the rectifiers and IGBTs in the spindle drive, and surprisingly, no shorted ones!
I think what I really need is a schematic (and or maintenance manual) for the NEC ADU75FP - someone has "been here before" and did a terrible job - beyond the obvious terribleness I could see initially. It looks like they replaced all the zener diodes ZD8 through ZD15 with 5.1V zeners which seems wrong. I think the gate firing voltages should be +15 and -10V. I don't see how 5.1V zeners fits anywhere here... of course on the board there's no indication of what value zeners should be there, and the original parts are gone, so it's going to come down to a schematic, or comparing to a known good, or comparing to a similar drive and doing some reverse engineering / educated guessing.
Some people should not be allowed anywhere near the soldering iron...
I could also really use the Seikos LIII/A Maintenance manual - I have the others, but not the maintenance manual that explains any alarms