mismatched background

hi guys, I film using stop motion, but most webcams are light, and move easily, especially on carpet.

I have footage of the main characters doing stuff, but it needs a background to cut out my hands, however, the two pieces of film are unaligned, and one is lighter than the other. I don't want to have to mask of the characters specifically, as they are very angular, and I don't want to have to shoot the background again, as the conditions are not the same, so I ask this:

  1. how can I get the two shots two match well?
  2. any tips for how to really stop the camera moving?