Which simple explanation? That the universes are all part of a single 4-D structure? That's quite assumptive with how our standards go, we generally deem universes as separate structures (aka, each one as a individual Low 2-C structure), add to that the fact that we do indeed default
there being a 5-D space between universes for the sake of them being parallel to each other, and in total an argument for 2-C is more than reasonable.
And so all of those assumptions you claim are either already a default that'd require evidence to claim otherwise for a given case, or just results from the very premise applying at all, it's not like this is an outlier when the character in question doesn't have much else to speak of.