I've been wondering about this and related topics as well. The obvious question is how a soul rejoining works: given that the Calamity apparently means everyone's souls on the Source got rejoined, and yet nobody mentioned feeling weird during that time five years ago. Specifically non-Eorzeans, whose interaction with the Calamity is mostly "the red moon descended then disappeared, and then lots of traumatized Eorzeans started showing up". (For Eorzeans, presumably the sheer terror of the Calamity plus whatever aether shenanigans caused the post-Calamity amnesia might be suitable handwaves.) Even as a non-Legacy player, we are clearly not merely five years old, so there should be some mechanism by which a soul can be grafted onto an existing one without the existing soul even noticing, as opposed to the lightshow when we joined with Ardbert.

And the other question, which you brought up, is how long each Calamity lasts, in cosmic terms. What is the "start" of the Calamity, and when does it "end"? It's likely that cosmologically, the Calamity "starts" when the barrier to the Shard breaks down and an uncontrolled aether transfer phenomenon occurs, but what does that mean in the case of the Seventh Calamity? Was it just during the very short period of Bahamut's rampage? Or did it start earlier, when Dalamud was in the process of falling? Both are possible due to time shenanigans between Source and Shard, but we don't know which case it is.