I received the new door switches and (for like the 20th time), removed the microwave from the wall and set it on the table to work on it. Got it apart and quickly discovered there are THREE, not two, door switches, though I had only bought two. Anyway, long story short, there's a great appliance parts store near my house, so I went there and got one more.
I put the new switches in - indeed, it was a frighteningly easy swap out - and reinstalled the microwave and turned the power back on. And.... success! Things are working great right now. Of course, it hasn't been a week yet, and I've only gotten a week or two out of previous fixes, so we'll see if this one holds. I will say this, though - the swapped out switches looked BAD - brown, burned-out looking contacts, all three of them on all three switches, so I HAVE to believe that was the culprit after I had tried just about everything else.
Anyway, I'll be back in a month or two to share any updates for those that are curious.