Here's where your problem seems to be. You think emotions are fleeting. This isn't true. Emotions are a response meant to move people towards positive things, and away from negative things. Purely a survival mechanism from an evolutionary standpoint. Therefore, it is possible to predict emotions. Whatever hurts the user evokes a negative emotional response, whatever aids the user evokes a positive emotional response. They are not random, it is in fact quite logical.Because Fi is simply a tendency to emote in relation to the inner life, it ipso facto lacks logical structure. (I anticipate some may remonstrate that there are logical Fi people in the world. I do not wish to deny that. However, they are logical not because of their Fi faculty but due to their competence with their Thinking faculty) Fi, unlike Fe that conforms to the rigid structure of the external world of values (therefore does not change easily, it changes in a manner akin to the whims of fashion, in small ways. We notice that significant traditions and values of the community rarely radically change, and hence neither does Fe for this reason.), has no solid foundation. It is founded on mere fleeting feelings. (There is no solid foundation because Fi in itself does not conceptualize feelings, it only processes them, hence there is nothing stopping Fi from going from one temporary passion to another.)
This is all recorded in the Fi users model. As more information is gathered and added to the model, it becomes more secure, much in the same way as a Ti model.
The Fi model is changed when a contradiction is discovered in it (in an entirely logical process).