If future Biff goes back to 2015 right after he gives young Biff the almanac, he could get back to the 2015 with Marty and Doc in it. Because it wasn't until his 21st birthday that 1955 Biff placed his first bet.
Wait. Whoa, whoa. Is "placed" right?
What do you mean?
Is "placed" the right tense for something that would have happened in the future of a past that was affected by something from the future?
Had will have placed?
That's my boy.
Okay, so, it wasn't until his 21st birthday that Biff had will have placed his first bet and made his millions. That's when he altered the timeline.
Yeah, but he had will haven't placed it!
Therefore, in the new timeline, Marty and Doc never brought the time machine...
W-Wait. Is "brought" right?
Marty and Doc never had have had brought?
I don't know. You did it to me. Oh, I'm going with it.
Marty and Doc never had have had brought the time machine to 2015.