Hi! Yes, you're correct. Stopping the Source, as a potential "solution," is meant to be extremely tricky to solve. I don't think there is any obvious "intended" path to solving it—I've heard stories from various GMs making the attempt—but you're totally right that anything that occurs as the direct result of time travel would, of course, get fragged out if time travel never occurred. That's certainly why, say, Mamadou has been unable to do it on his own so far.
I'm a big fan of these kinds of open-ended "solutionless" problems, both as a player and as a designer. It forces the player to engage with the problem head-on, rather than merely hunt for what I as the designer had in mind. A rawer, one might say almost more brutal, form of play.
Hope this helps!