Since there was just one sea that ended up being split into 14, I can't help but wonder if the Ascien's "Rejoining" is actually then literally rejoining the these seas together? That they can only rejoin the seas once every Umbral era?

There have been at least 6 successful rejoining (as stated by Elidibus) and that the 7th one has always failed due to the WoL. I imagine that the 7th and final rejoining will merge these "seas" and Zodiark with Hydaelyn; effectively making Zodiark whole again