There is a simple and easy alternative by which to avoid any questionable behavior. Namely, don't eat snacks during the movie, and enjoy them before or afterward. It's certainly better under halacha to avoid potential violations when possible.
When you enter the theater you are agreeing to abide by their rules as their guest. Bringing in food, is a violation of that agreement. So right there you are obligated not to violate agreements entered into voluntarily by you.
Whether the overcharge is halacha, isn't relevant to whether you can bring in candy to work around it. For instance, if someone else violates halacha, that doesn't imply in any way that you can. One can't judge easily if the overcharges are essential to making the movie profitable enough to stay in business, so it may not be an overcharge after all.
Whether kosher snacks are available is similarly not relevant. You can choose not to eat. So by bringing in and eating, you are choosing to break your agreement made as guest in their business.