I have not attempted any reformat and reinstallation: I cannot take the time for it. I have just connected the old mouse and thus far this seems okay. I will have to use it for a while and observe the results over time. I doubt this would solve the strange keyboard behaviour. If it does, then... heh. I will post another time (maybe a couple days) to see if any erratic behaviour occurs.

Another thing I must say is how "refreshing" this old mouse is. A genuine old PS/2 with two buttons. Well, at least it's not the standard Mac that has only one(!)

I had also found this link which looks useful.