I think you are going to much into fate has "things are already determined", when Aladin is talking with the dark priest it seems to me that what is really being talked about is mortality. The flow of Rukh is the classic flow of life, people die and people are born and so on and the Rukh flows through it. Basically, we are all fated to die. The opposing party doesn't seem to like this at all (which I can also find understandable) and is actively trying to oppose the flow.
There is also the extra layer of the presence of the Magi like some sort of divine arbiters but even then we are seeing that it's not exactly "magi decides king, everybody sucks it, end of story", we are seeing how a guy that seems to be an amazing king and a great candidate to go even higher looks a hell of a lot shiftier than one would expect and another whose first act has been to found a republic which negates any sort of royal or divine right.
The chosen of the magi don't need to be rulers, they can also be guides to greater things which is the approach Aladin and Alibaba are going with.