When did old Biff fade out?
marty prime is a step above trailer trash. He plays in a band with old guys (That was the News pretending to be his band the Pin heads right?). NuMarty is probably a preppy and a yuppie who is smarter and more positive and more emotionally put together well than Marty prime because he wasn't brought up by POOR wallflowers.
It took a week for Marty to kill himself by cancelling his parents first kiss, and even then, because time had to be really really sure that every thing was gong to change, and as long as there was a fraction of a percent of a chance that the kiss would happen Marty Prime was allowed to survive even though many other events had conspired to create NuMarty who was a completely different person because of his completely different upbringing because he had completely different parents becuase they ad lived completely different lives.
So does that mean that old Biff was able to return to his home 2015 because time hadn't changed it's mind yet? Time waits for man?
I think it was NuMarty in the past who had made his own DIFFERENT changes to 1955 that was creating the potential for Marty Prime to exists without ever having being born and only if he gets things back on track is Marty Prime going to vanish and his birth and future history going to be assured if he too accidentally managed to play Havok with his own conception. NuMarties Timeline was staying intact for a proportionate amount of time to whatever bollocks was happening in the past in a holding pattern until it was sure which way it was supposed to unfold.
Really if anything it's marty prime who is going to va... I've been through this. Possibly I'm phrasing it a little better?
Jennifer met Jennifer and they didn't phase, Marty got pretty close to meeting himself and didn't phase, hell biff beat his younger self with a stick and they didn't phase into each other too.
There is no evidence of corporeal integration.