I've mended something! Unseasonable warmth meant the excuse to drive the 900 around to air it out a bit. It was also an opportunity try to catch any problems that arose due to it sitting. I discovered that 5th gear was incredibly vague to engage. It was almost like the lever headed towards an imaginary 7th gear. I'd already noted that the collar under the shift knob didn't seem to be required to enter reverse. As it turns out the two things were related. A parts diagram clued me in to look here.

The plastic guide plate for 5th gear/reverse lockout had become dislodged.

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This is how things should have looked.

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Here it is in reverse. Note the black tab lifted over the plastic guide plate. That's apparently what the reverse engagement collar is connected to. Lifting the collar allows the tab to clear the plate.

Airing it out in 60F weather after lightly treating the fabric with some mold killing spray seems to have helped clear some of the funk too.