How do calamities work, again?

[Cilia]

We know that the barrier was cracked on the Source because it was stated that the thinning of aether, which was previously stated to always preclude a Calamity, was a result of the Light seeping in.

Yes but this only occurred during the tail end of Stormblood (Shadowbringers' prologue, essentially); the Flood occurred a century ago on the First, before the Warriors of Darkness sojourned to the Source in an effort to stop it or cause a Rejoining so their existence wouldn't be wiped out entirely. Minfilia's intervention halted the Flood, but the elemental imbalance on the First was still tipped dangerously in favor of Light over Dark, which would have led to the Eighth Umbral Calamity (Rejoining) if not for Crystal Exarch G'raha's intervention.

The barrier isn't cracked during Heavensward, which is when the Flood was an immediate threat to the First (as opposed to the Sin Eater infestation we deal with in Shadowbringers).

People keep saying the Flood here was counterproductive to what the Ascians want, but I don't recall that ever being said. Does someone have a source for this?

Elidibus mentions it at some point during 2.1 - 2.4, during the Warriors of Darkness' mini-arc in Heavensward. I don't have the exact line of dialogue, if that's what you want, but it's there.