pixel_player69736434 I think I figured it out.... I had also been back through all of the levels and searching, and double checked the clues given on here, and finally figured out where the last key was.
In the professor's cabin during the first go through, we put the balls on the mouse tracks near the potato sacks. When we go back to the professor's cabin, the last key (that I found) was in the same spot. There's no balls to move onto the mouse tracks, but one of the potato sacks had the key. Just clicked and the key fell out.