you can thank CBR for this plot twist idea...
picture, if you would. doc brown standing on the toilet trying to change a Burt out lightbulb as Setsuna. Steps out of a portal to bean him upside the head, and leave the ideas for the flux capacitor behind... Scene shift, Marty finds his way to doc in 1955 thanks to The trick Setsuna plays on biff at the soda fountan... And it goes from there to the end of 3 and the cartoon adventure until she has secured the way for Usa to come to the past and harass ...Ranma...
((Who ever said the key Usa uses wasn't the key to a time traveling deLorean......))