I’ve just seen the movie; I haven’t read the story it was based on yet. I had no trouble following this but it probably has references I missed.
I agree with cbhacking above. About not using precognition: trying to change things is one of the first thing you should do once you’re not in a high-stakes situation. Others include information generation from nothing, but we already know that works.
With the acausal time loop, it would be fine if she’d said sometime in the intervening eghteen months, “This is weird but you need to tell me at the party what I said to you,” but he says at the party that what he’s doing was just a hunch.