Still, why were the Prophets suddenly aware of the idea that Bajor might be destroyed just then?
Again, the problem is that you keep using words that are rooted in our concept of time. "Suddenly" is not a word that has any meaning when talking about timeless beings.
After all the time that passed, they are suddenly convinced just then that it could happen--
Because for them, no
time passed, and every moment in the entire history of the universe was "just then." You need to set aside the very concept of time passing, of things happening in any order or at any pace, in order to grasp how the Prophets think or how causality works for them. Just think of how one event causes another. Ignore our perception of their relative order or separation. Just think of the progression of cause to effect without bringing time into consideration at all.