i don't think there is any real reason besides making it easier to connect the batteries in series, but thats just my guess
I think they do it in parallel so they can get away with it. I remember my wireless mouse worked with only one battery meaning it was in parallel but why they do that I don't know
the way you insert it really doesnt have anything to do with how they are wired.It is likely that the mouse is wired in parallel, though.