From what I can personally confirm, this is how the region of Pulchara works.

It appears that you need to meet 1 of 3 conditions in order to be immediately recalled back to the central tower to end the scenario.

1) The calamity overtakes 3 regions. Note that the calamity progresses by one stage every time you do something in a different region, a 'red' event means that you run the risk of the calamity overtaking that region if you do something elsewhere. I do not think it is possible to prevent 2 or less regions from being fully overtaken.

2) The calamity stalls within the region whose plot thread you start (as in, White, Yellow, or Black). If you manage to fully dispel the calamity from two other regions in the meantime, you may be forced back to the central tower early. Some of the plot threads, especially Black, have events where you have to spend a 'turn' in another region first before the next step of the plot thread appears, but if the calamity is already fully dispelled from the other regions yet to be overtaken in the meantime, the game will detect that there is nothing for you to do within the other regions and immediately recalls you back to the central tower.

3) You finish your chosen plot thread.

Ameya will join your party if you decide to fight against the Final Calamity yourself, with one major exception. If you complete the 'Yellow Witch' plot thread, a different mage will join your party, and attempting to fight against the Final Calamity afterwards will result in Ameya ignoring you to fight it herself anyway, and failing.