@zitqhile
Was a main female character an enemy in the past life and became friends/more friendly when he went back?
Absolutely - the MC recreated the band he was in at college for a talent show, actually one of his ex-bandmates suggested it, but it required a change in lineup to meet the entry rules so the MFC was brought in. She pulled a fast one with the paperwork, and walked off with the (2nd place?) prize money, which was what triggered throwing the bottle. This also coloured the MCs attitude to the MFC after going back, which took a while to resolve. Life rolls on, and eventually they reform the band for the talent show again, this time winning it.
ETA more snippets: The MC throws a temper tantrum on first remeeting the MFC and trashes one of hid frind's drums, an action he later describes as 'a dick move, of John Holmes proportions'; the reconciliation begins when he comes to the aid of a female student who is being assaulted and it turns out to be the MFC, this prevented assault having been the start of the MFCs slide downhill to the bitter and twisted character at the beginning; 1st time through the MC was an only child, 2nd he had a brother and sister; the MC has a dream of the time the timeline split, showing the MC and MFC meeting as infants (shades of Jay Cantrell's Learning Curves, but even younger) and explaining the siblings anomaly. A band member of the talent show displaced winners is also a returnee, and clues in the MC on the importance of keeping the bottle safe.
I'm going to have to reread it too.