I mean, clearly we‘re not dealing with a fictional universe where there are multiple time lines, but what would have happened if Sheridan had told everyone what he saw in the future, speaking just in Watsonian terms?

I think that what happens in Babylon Squared and War Without End mean that it has to be free will rather than fate - Sinclair sees the timeline where Garibaldi boards the station and dies and as a result prevents that timeline from happening. Unless it's fated that Sinclair sees the future and thereby changes it, I guess?)

(If things are not fixed, it means that Londo not only missed four chances to change his fate, but also that he could have missed the redemption boat entirely. One shudders to think.)