meetings can be outside of availability, but if you absolutely cannot make it then you can’t. but i’d assume it’s pretty much impossible to schedule a meeting that meets everyone’s availability, so it makes sense. otherwise yeah they’re mandatory

Your manager doesn’t need to know what you do in your off time, the only reason you would need a “proof note” is if you have a doctors note saying your appointment was then or even your new limitations. Your manager doesn’t need to know and shouldn’t ask that of you. Honestly stand your ground and put up boundaries. That “mandatory meeting” is happening in every Starbucks across the world, all that you are doing is watching Howard do a speech then you as a store will talk about the speech and your manager will answer any questions, most of it will be about unionizing and weather or not you as a store will unionize- which I would recommend with your terrible management situation. As a unionized store you won’t take shifts at other stores and your scheduling will be more concrete. Id recommend looking into it and contacting a dm