I think you are saying that the top-level could be modeling a completely determinstic world that could be anywhere from slightly to completely different from th

I think you are saying that the top-level could be modeling a completely determinstic world that could be anywhere from slightly to completely different from their own? That's a good point, and it would mean that the first simulation need not see the viewport pop into existence. They could not then assume they are the top-level, however (which would be false), and I don't think they could conclude "either we are the top-level or the first simulation", as, if the latter of those possibilities is the case, they cannot rule out the level above them also being some sort of simulation, only that theirs is not an exact replication of it.