Thank you! Someone gets what I've been saying for decades! Biff created a whole new timeline when he gave himself the book, therefore he should not be returned to the original timeline. He should be returned to the future of the new timeline he created.

And true, how would he know the specifics on how to setup and activate everything.

He was unconscious and she's already seen him in his underwear since she starts calling him Calvin Klein before the mirror scene. And since she'd already looked, what else did she peek at when she had the chance? She's rather bold and forward, I wouldn't put it past her.