Crackpot theory inbound.
So there's four universal instances, but that doesn't neccesarily mean that there are four universes. Universes, as we all know, are pretty damn huge. Yet in each session we only focus on the inhabitants of one single planet. Universes inherently contain a lot more planets than that though. What if an Earth and an Alternia exist in the same universe? I personally believe that the new human session-players exist on an Earth that is in the original troll universe, who in turn are communicating with trolls that exist on an Alternia that originates from the human universe.
Looking into it, there is some supportive proof of this. Disregarding the obvious Mobius-Double Reacharound, there's also the formerly established connection between the Betty Crocker brand and the Imperial CondenscewhatshernameIcantspellitright's flagship. We know the trolls are capable of interstellar travel. Despite the earlier-mentioned mindnumbing size, the possibility exists that they eventually reached Earth and influenced it somehow.
On that note, "Scratching" a session basically restarts it, while flipping around the roles of the ancestors, at least if I've understood it correctly. What if the restarted session took place in the other universe? It's implied that the kids are the guardians of the players of the new human session, atleast if SBaHJ the moive is anything to go about. The kids of this new session are communicating with troll kids that are the pre-scratched version of the ancestor trolls that originally exist in the human universe and...
I fully expect this theory to be undeniably shattered by staggering amounts of evidence, but it is there for your complentation nonetheless.