"At-will" employment states have exceptions so that you can't fire people on the basis of race, gender, sexual orientation, age, religion, disability and so on. California adds political expression. They take precedence over "at-will" termination, not the other way around.
Sure. My opinion, based on the filed lawsuit, is that she has a good case, not that her firing was wrongful or that Disney won't have a good case, as well. Before this week, I thought that Disney had the right to fire her. Now, I'm not sure.
I am pretty sure that repeatedly warning her won't matter if they weren't allowed to fire her for it in the first place, though. Even warning her against political expression is against California labor law.
I imagine that their defense will be either that those sections of the labor law don't apply in this case (i.e. that her posts don't qualify as "political expression," as you suggested) or that they fired her for other reasons, such as declining to participate in the sensitivity training which they wanted her to attend. It might be hard to argue the latter, though, because the announcement of the firing indicated that it was because of the content of her posts.
I'm interested to hear Disney's side of the story. I generally support the ability of companies to fire without cause and don't necessarily agree with all of the restrictions on that, but the law is the law and, like you, it seems, I'm open to whatever the verdict ends up being.
BTW, if you aren't sure what she posted, you might care to spend a half hour reading through the lawsuit (which I linked on the last page). It's a surprisingly easy read without legalese and with lots of screenshots. Yes, it's her side of the story, but we should want to hear that and it's a good summary of what she and others were saying on social media.