I personally don’t think she’s has an immortal lifespan, longevity yes! & will most likely out live her friends (unless some other official media makes mention why Celestia lived for so long), but not immortal.

Although I agree that Starlight will use the Time Travel spell to stop the Rainboom from happening I doubt Starlight will have taken over, considering she would then have to tend against Nightmare Moon, Still Evil Discord, Chrysalis, Sombra, and Tirek. I’m guessing in this new future Starlight may end up being a freedom fighter helping Twilight to fix things.
 
I’m guessing Twi will put up some sort of defensive spell around her and Spike before Starlight uses the Time spell that ends up delaying the effects of the new timeline on them. Though I do expect for Twi to start reverting back into a Unicorn, old memories being replaced by new ones, and her watching as Spike slowly fades from existence.
 
I can already guess how everything is going to be returned to normal. Since in this new timeline Twilight never used the One Time Time Travel Spell in “It’s About Time” she uses it to go back and stop Starlight from stopping the Rainboom.