... I wonder if the lower right screw that was behind the port cover, that was so much harder to get out than its fellows, damaged the problem screw. It might have been too long and impinged on it.
What a pain in the patoot.

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If the screw got partially stripped by another, the stripped section when backed into a nut can spin with the remaining good section of screw acting as sort of a loose rivet head. So tightening such a screw will often properly tighten, while backing it out starts to work then stalls and spins on the stripped section. It's sometimes possible to start the remaining good screw section threading if the head can be strongly lifted relative to the nut while turning counterclockwise, but there is the risk of jamming and/or stripping the nut if crossthreading occurs. Tough call whether to tighten the screw then drill it out, or try backing it out with simultaneous lifting force. If the screw will not even tighten properly and just spins, it might have to be glued in place just to drill or grind it out.