What aligments would future calamities even have had?

[Seraphor]

I don't believe the balance of aspects for the calamities has anything to do with overall aspect balance on the Source?
It's not like a Flood was required because the Source was too low on water aspected aether, the Flood occurred because one of the shards was overaspected towards water, making it a prime candidate for a rejoining, and so a Flood was orchestrated on the Source to facilitate that rejoining. That Flood was in turn amplified by the rejoining of predominantly water aspected aether so in fact, it could be argued that the calamity tipped the scales too far towards water, for a time. The Flood waters receded, implying that aspect imbalance had righted itself.
Aspects aren't immutable, they change and shift, that's HOW shards end up becoming overly aspected one way or another, their aspect changed towards Water, until it was too much. So the same can be said for the Source, that aspects can rebalance.

The First was already overly aspected towards Umbral aether, prior to the plan to cause an Umbral calamity ti trigger a rejoining. So the shards aspect is what decided the form of calamity.
So any subsequent calamities would be aspected according to the aether imbalance present on their respective rejoined shards. If Shard no. 9 was a world overly aspected towards Earth, we'd get another Quake-related calamity.
The Source would then become overly Earth aspected in turn, which would BE the calamity itself asserting it's effects, until that 'Umbral Era' ended and the balance returned to normal levels.


As for not needing any further calamities. It's an assumption, but it's not an unreasonable one. The Thirteenth was written off, so we already know that it's not required for ALL shards to be rejoined, a partially rejoined Source would be sufficient. It's just a question of how much would be sufficient, and with no other info to go off, >50% seems like a sensible assumption. You go above 50% and you're tipping the scales more one way than the other, so half of the worlds being rejoined +1 would be a decent prediction of what would be sufficient to free Zodiark.

[Iscah]

The Thirteenth was written off, so we already know that it's not required for ALL shards to be rejoined, a partially rejoined Source would be sufficient.

We still don't know for certain whether it was not required, or simply put in the "too hard" basket while they work on rejoining the other shards.

For all we know so far, it's possible that they were manipulating Golbez into eating the local Zodiark shard and then breaking through the barrier into the Source, bringing Zodiark's missing piece with him. Or maybe that shattering of the barrier would be enough to hook the Thirteenth back up to the Source's lifestream. Those are certainly the sorts of plot holes I'd be trying to fill as a writer at this point.

[WhiteArchmage]

We still don't know for certain whether it was not required, or simply put in the "too hard" basket while they work on rejoining the other shards.

For all we know so far, it's possible that they were manipulating Golbez into eating the local Zodiark shard and then breaking through the barrier into the Source, bringing Zodiark's missing piece with him. Or maybe that shattering of the barrier would be enough to hook the Thirteenth back up to the Source's lifestream. Those are certainly the sorts of plot holes I'd be trying to fill as a writer at this point.

Or that the Ascians were tempered as admitted by Emet himself and their directive was to rejoin and free Zodiark no matter if that was actually possible with the thirteenth "lost". We DO know they tend to act and justify themselves with the sunk cost fallacy...