I think it really needs to be rewritten so that the kithrak and dragon fly off after flaming the Flaming Fist and it’s another character who you speak to, who is then part of the fight should it come to that.
I don't know. I think the whole scene works fine. They just need a MUCH better reason for him to leave.
That would also work, of course! I just struggle to think of a reason that’s not going to feel contrived and/or overcomplicated, so suspect rewriting might be simpler. But I am open to being wrong on that point!