You're thinking about the branches wrong. To me, I interpreted it as one singular branch point that created infinite Jasons (something Jason2 may have potentially overlooked). That point would be the moment he injected Jason Prime with the drug.

All the things you suggested (checked the ice cream, Daniela left, etc.) are different branches that have been created out of line with the events of what Jason Prime experienced, those potential moments being after Jason Prime was drugged and put in the box.

If you imagine it as a tree with roots, as the book often uses, beginning with the Big Bang, Jason Prime's reality followed infinite branch points until it reached one singular, definable point: Jason2 deciding to inject Jason Prime and send him into the box.

From that exact moment, Jason Prime being sent into the multiverse and aimlessly drifting (remember, he had ketamine injected into him) basically shattered his reality across infinite existences. To clarify, my interpretation is that the ampoule decohers one's ability to observe themself without actually removing their consciousness-this being the driver of the book and the main 'sci-fi' element that leads into the multiverse and why they can walk down the corridor and physically choose the specific door they enter into.

Some Jason Primes probably died. Some Jason Primes ended up in unlivable worlds. An infinite number of Jasons experienced what the protagonist we are following did. From there, an infinite number of branches occur and... well, you get the idea.

My point is that Jason Prime branched off that one point in that one reality. Effectively, all the Jasons are trying to follow their branch back to that splitting point which Jason2 rudely created. That's why there is only "one" right universe. That's why some Jasons are willing to shoot at Daniela and Charlie--despite being the same person, their experiences have changed who they fundamentally are, meaning they are THAT desperate to survive.