Well, after nearly seven years, he was looking a bit worse for wear, so I hit the internet and bought a second one. It took a few weeks, during which time Numfar would move both Misters Mouse, one at a time, whenever she wanted them somewhere new, but eventually she started to prefer the new one and we retired the original Mr. Mouse for good.
One day, though, I came home and discovered that she'd apparently felt that the two Misters Mouse needed to be separated and set each down in identical poses, almost exactly equidistant on either side of the corner in the hallway. Sometimes, I really have to wonder what goes through her brain.