One question that was never answered, because it never came up is what would happen if a second person went down the rabbit hole during that brief period time while the first was still in there. I expect someday the premise of the rabbit hole will be revisited and that question will be at the center.
Three possibilities exist there, I think: 1) The first person would get erased as the second person entered and time reset (Person One would simply poof out of existence along with that reality, creating a very curious little case in their own time); 2) Person One and Person Two somehow inhabit the same time frame, i.e. time resets but treats the presence of Person One as an established fact; 3) time resets only for Person Two, meaning that Person One is in a different timeframe and therefore alternate universe.
On a quantum level (multiverses, anything that can happen does happen and co-exists within a different time stream that you can't access and therefore know nothing about), the chances are that all of these things happen. Other timelines are then formed due to cause and effect, while the original one also still exists (and may reassert). E.g. you find the rabbit hole in what you perceive to be your reality. At that point two new universes come into being. In one, you go down the rabbit hole; in the other, you no 'Nup' and walk away (multiple new universes spring up at that point: in each reality a different direction is taken, then realities branch from that depending on the speed you walk or run at, then other spring up around 'do you tell anyone? Y/N'. And so on. In all realities direction and pace lead to other consequences. In some you might have a near miss, in others you get mown down by a truck (in others it's a car, or a meteorite lands on your head, etc. Anything that can happen does happen, remember?). And in half of these alternate universes/realities you're of the opposite sex, because way, way back a certain chromosome formed - or didn't. Anyone got a headache yet?
)
So in the book, after changing history (in reality, you would simply switch from your own perceived reality, which is both past and still a possible future to you at the critical point, to a parallel track, whereafter you can't get back to your true reality without finding a rabbit hole and doing it all again, letting events play out in certain ways), the nightmare future would still only be one
possible future. It's just that it's the one from the infinity of alternatives that he's aware of.