I have thought about the possiblity of Alduin being weak due to this Kalpa being different with him trying to rule the dragons rather than just do his one job. It's a theory that has been circling. Others say that Alduin's power or strength grow over time until they reach their maximum potentional, in which case he begins to devour the world. In this Kalpa, his desire to rule made him power hungry, which in turn was the reason for him starting his world-eating early. However, how much of that is true, we don't really know. We do know that something about this Kalpa was different enough to make Alduin jump the gun and try to rule the dragons. Maybe he got bored? After devouring countless Kalpa, maybe he thought he was some sort of unrivaled powerful being who deserved more?
With Mundus gone, the other planes do still exist, The Void, Oblivion and Aetherius. Oblivion is the only one people aren't entirely sure of since it's supposedly, like you said, another plane of Oblivion. Since the three other realms existed before Mundus, we can assume that Mundus's sudden inexistance wouldn't effect them at all. The only reason I can think of as to why Oblivion doesn't need Towers, yet Mundus does, is because it might have something to do with Mundus being created rather than naturally existing. Either that or the fact that it is literally made from the power/energy of gods, mostly that of the Eight Divines. So made all that power that Mundus is made out of is unstable?