Until now, I decided to do it proper and pulled the logic board out. Good news is as far as I can tell the logic board is fine, buttons are fine and responsive. I put it in the case and buttons feel much better now. I suspect the logic board's latch to hold it in place wasn't latched properly by the previous owner and the board drifted forward just enough to cause this issue. But I'll have to leave it until tomorrow before I confirm that theory.