That doesn't work. If the timeline had already changed by that point, there wouldn't be a time machine. When they go back to "Hell Valley" 1985, they find out that Doc had been institutionalized and never got to test run the Delorean (And probably never finished it) Doc and Marty should have faded out too.

I feel that deleted scene actually hurts your argument if we take it as canon. Like you said, the disappearing fadeout stuff happened to the siblings in order of oldest to youngest. This Biff came from 2015 and logically he should survive the timeline change longer than 1985 Doc. Doc was born before Biff and comes…