Ha, I didn’t think about it much. I think Alex’ returning might have helped - because I think Alex returned in the prime timeline, told Charles that Raven rescued him and those other two guys from Vietnam, that she was alone at the time, which helped to break him out of the worst of it. Alex might have been stable enough not to need much help, but I think other students/teachers returned from Vietnam with severe PTSD and Charles picked himself up enough to help them.
The school took a little longer, but that’s how it started, people who needed his help started showing up. He might have rejected them initially, but some of them wouldn’t have had a place to go to, so if not Charles than Hank would have told them to stay, just not to get in Charles’ way, but Charles has a hard time not helping people in pain.
Me, I assumed that Raven got captured by Trask and Charles found out about it and kicked into gear— detoxed from the serum, and got Hank— and Alex if he was back— to help him infiltrate and rescue her. Actually, that may also be how Erik escaped the Pentagon in that timeline: Charles reluctantly broke him out in order to get his help to save Raven. A powered Charles could do the prison break with a lot less fuss.
So Charles and whoever he dragged into it went to save Raven. Except that the moment their attempt gave her the slightest opening, Raven took advantage of it and was already long gone before they could ever reach her. She was so traumatized and Charles was still re-mastering his powers, he couldn’t risk delving into her mind to find her again. So it was Erik who caught up with her, and helped her reclaim herself, and strike back, and never stop striking.
That’s pretty close to my headcanon too.