Back on the subject of BJ, I don't think he's a junior at all--I think he's just Bowser from the past. Let's look at the evidence: Bowser never identifies his younger self as himself. Baby Bowser looks exactly like BJ in PiT (which I believe takes place after YIDS(both time lines)). In YIDS, Bowser is shown completely capable of time travel on his own, or he used E. Gadd like how BJ used him to get the "magic" paintbrush.
BJ is prepubescent Bowser.