For anyone interested, I did finally fix this. Replaced 3 door switches to no avail. Further checking while it was apart revealed that the door wasn't closing 100%, so one of the door switches (interlock #2) wasn't engaging. Top part of door looked spread slightly, which seemed to be what was keeping it from working. Maybe a result of taking the door apart repeatedly to replace the torsion springs. I finally just dremeled the inner edge of the two latches on the door so that the rotating mechanisms that the microswitches sit in could turn an extra mm further. (a little counter-intuitive because the latches first turn the disks inward, but when mostly in the springs push the disks in the other direction so that the latches are then keeping the disks from turning further.) That pretty much did the trick. Not perfect, but the switches do engage 95% of the time when closed. If not and the light's on, a light push or tap is enough for it to register as closed.
PS. As I was trying to fix this, the MW changed to beeping error that the door was open, where before it would actually turn on but magnetron wouldn't engage. With a little searching I found the Buss fuse had also likely failed, probably as a result of my tinkering. Another trip to hardware store for a $3.50 part and it's back to working!