"Jim Conway" wrote

1. Are pre-finished stair treads (for interior staircase) common and
something I would be able to find easily, or do they typically sell
them as unfinished? I'm looking to avoid the toxic aroma associated
with finishing them in their entirety if it can be avoided.

Yes, but may not be at the box stores where you are. Google will show many
of them you can order online.

2. Present plywood treads are glued/nailed to stringers. To remove
could potentially damage the stringer. Is it feasible to overlay the
new tread directly onto the existing plywood tread? I realize it could
skew the height of the top or bottom stair by the thickness of the
tread....

Actually that will work fine but be sure you do it with all of them. Like
the other person said 'feet remember'. It's ok if the bottom step is a bit
different, but the one's between should be the same.