Beo,
I disagree that seeing the future changes it.
No problem, I had my sneaky bastard GM hat on. That's the way I'd fudge and balance it, by selling it that the players have too much agency rather than none
If this was a normal story, I think playing with predestination as a solid construct to fight against is more fun that instant fluidity. I've done so with prophetic visions in prose I've written before.
For the sounds of your story, I think that you need to have each vision not just a random one that is useful to the player. Well, you can do that in combat in the way suggested, perhaps as a tactical choice of whom gets focused on and gets a small but meningful bonus.
Outside of combat, a vision should implicitly grant a responsibility. She gets a vision of someone doing good and being in power. There';s almost an automatic 'do you let this happen' due to some aspect of the vision. After that, she gets further visions when interacting with other NPCs and PCs that show the necessary steps for that first vision. Each time, there should be a dramatic implication - alter the timeline or defend it.
Basically, have the players choose to railroad themselves or not, and have both paths be interesting. It's up to them if they mess up your constructed story, and so you have to worry less about justifying why things aren't going the same way if the push against it.
Throw in some arbitrary visions to do with player conflict - she seems herself stabbing one of the players. If that player later mucks up the timeline, you can retcon that her choice earlier on something directly led to this invalidating of the future.
Or, of course, when another player is going to do a vision breaking action, pause the action, have the vision of the alternate future flash in her mind as a retcon, and then see if she blocks the player or lets thing happen.
