I have both organic Mastic and premixed Thinset left over from the tile
job I did on a bathroom shower. Can I use the remains of either of
these to patch and fill holes in the drywall (not cement board) above
the tile? It seems to spread like regular joint compound......just
wondering if it sands well when it dries and is "paintable".

I imagine it will be very hard to sand.

No, you won't be able to sand it smooth.