Appointments outside of memory care are becoming more challenging. How would you handle this?

My 91-year-old mom lives in memory care, and from time-to-time it's necessary to check her out from MC to take her to doctor appointments, etc. Months ago I'd call her the night before, tell her I'd pick her up at 9 AM for a 9:30 appointment, and when I arrived on time the next day, she'd be ready to go. It's amazed me that despite her deficits Mom's been organized enough to manage this; but still, in the back of my mind I knew the day would come when she couldn't. Sure enough, I began to notice that when I'd arrive, she'd know we were going somewhere but still not be dressed, and I'd have to prompt her to get ready. Entirely understandable. However, more recently, I've found her ready to go but furious, insisting I'd told her I'd pick her up at 6 AM for a 9:30 AM appointment, saying "Where have you been? I called and called and you didn't answer!"; essentially a catastrophic reaction with anger, hand-wringing, pouting, refusal to leave, etc. Reasoning, of course, does no good. In both cases it took a "time out" followed by a great deal of wheedling to get her out the door. Fortunately, we weren't late for her appointment. Obviously, I need a strategy to avoid more of the same. I've contemplated arriving early and unannounced to assist Mom in getting ready, but I can hear it already: "Why didn't you call me??? Nobody tells me anything!!!!" with another scene of anger and confusion. I've tried to limit outings with Mom as much as possible for my own sake (there's a long history of verbal and emotional abuse; thank you, Mom), and my sister shares in my care-giving duties; nevertheless I'm anticipating a number of future outings, and without going into detail, they are necessary, non-optional followup appointments and social engagements. So far Mom's social conduct is okay, sometimes inappropriate (but not combative) and so far I've been able to manage her okay. The staff at Mom's MC love her and are wonderful; however, they don't have time to coach residents about when to get ready, etc., and they don't prepare residents for or take residents to appointments; just a once-weekly group pleasure jaunt on the community bus. Has anyone encountered similar situations and how did you manage them? Thanks!