A show that features time travel has to feature a level of consistency.
Vincent is Traveler 001, and Simon is 004. The three-digit numbers show that in the beginning the program hadn't conceived of needing thousands of people. But they'd figured that out pretty quickly, since Grace is 0027.
The revelation in "Omega" that the final cause for the Bad Future was a nuclear winter brought on by war over diminishing resources hints at just why the future stubbornly stays the same no matter what the Travelers do, and also why the Grand Plan seems to call for fixing such a wide variety of problems. "Diminishing resources" can be caused by any number of factors, so unless you make sure that none of those factors remain, the war will come, the winter will follow, and humanity will end up in that same crumbling shelter.
In S2E5, we see a previously overridden consciousness of a host overridden by another consciousness, suffer a nose bleed. In S2E7, we see what happens to a host that is consistently overridden, seconds after each other in real time. So after the reveal of S2E11 and what happened to the original Marcy, what does that mean since this she's on her third override?