How could their fate affect the fate of the natural disasters?

Great question.

I don't remember it being explicitly addressed, so that's another unanswered one.

But, according to my husband (who was able to pay better attention to this film than I was, as I was wrangling our baby during the movie), anyway — so, my husband noticed that they said that the catastrophes were triggered by their deaths.

Oh, and they'd stop having their nightmarish visions if they could prevent the catastrophes from happening.

But it didn't address what would happen if they let things play out as-is, without intervening.

You'd think that if they knew their own death would trigger the next disaster, that they would withhold killing themselves?! Or no?