Reverse engineering is not really something I know too much about.
I mean, my gut feeling would tell me to first check the different chips on the board, to see what they do. You can usually found out quite a bit that way already.
Although most functions of the mouse go through the microcontroller probably.
This video might be interesting to find out the basics of how a mouse works:
Keyboards is something I am really enthusiastic about, which is why I wrote that topic. Mice is something I am much less in to, so I don't know if I would make a topic about it.