What saved senpai from that distracted biker (?) in the original timeline?
She probably got hit and still lost the phone. Maybe she was laid up for a while and that slowed her progress in acting in school. Then she was really too busy for romance, trying to catch up.
Also, since there are shout-outs to "Back to the Future," well, the first thing that happened to Marty McFly was pushing his father out of the way of a car that would have hit him (causing him to meet his mother in a love-potential way), getting hit himself and having his mother fall for him, so maybe some guy would have saved her.
Actually, the scene itself provides the context clues. In the scene Seto is leaning against the wall next to the door as she talks on the phone. Nanasawa decides to walk by and get a closer look at her, and in the next panel Seto has stepped forward and is walking.
She could've seen Nanasawa approaching out of the corner of her eye, thought she was moving toward the door of the cafe, and stepped forward to move out of her way, the phone call distracting her from noticing the approaching bike.
In other words, in the original timeline, she just stayed where she was for a few seconds longer and was never in the path of the careless driver.
(I mean the real reason is that the author probably didn't think about it, just like they didn't ask themselves how Nanasawa has working cell service in the past.)