I felt a couple of times that this is trying to separate a combined thing into two concepts. But possibly it is in fact two things that feed into each in a loop. For example, in your Avatar example, I hear Katara as pretty clearly making an epistemic/​prediction claim. But she is also inviting me in to believing in and helping Aang. Which makes sense: the fact that (she thinks) Aang is capable of saving the world is pretty relevant to whether I should believe in him.
And similarly, a lot of your post touches on how believing in things makes them more belief-worthy (i.e. probable)
I felt a couple of times that this is trying to separate a combined thing into two concepts. But possibly it is in fact two things that feed into each in a loop. For example, in your Avatar example, I hear Katara as pretty clearly making an epistemic/​prediction claim. But she is also inviting me in to believing in and helping Aang. Which makes sense: the fact that (she thinks) Aang is capable of saving the world is pretty relevant to whether I should believe in him.
And similarly, a lot of your post touches on how believing in things makes them more belief-worthy (i.e. probable)