Now, onto said other stuff: You seem to try and justify the idea that the Nine Realms having uneven flows of time makes them separate spacetime continuums with information that says you can't traverse them through conventional methods of transport, but currently our standards on this are:
The Worlds being different bodies of space is not enough to consider them universes, as that does not quite prove they have different timelines. Especially if they are afterlives. Afterlives often are different bodies of space, but are generally condensed in the same timeline unless there's more proof otherwise.
So, if worlds being different spaces isn't evidence of them being alternate spacetimes, and neither is them having different flows of time, I don't see how those two in conjunction would be, either.
That, and the fact that Ymir was explicitly born "when time was young," tells me that the Realms just share one dimension of time. I think assuming that this just so happens to refer to some higher temporal dimension is pretty absurd leap without more evidence to substantiate it, especially in the presence of other interpretations that are just as logical.