OP - are you always a wet lettuce like this? So many PPs giving you good advice, and good clear messages to send your MIL, and you keep replying as if she's magically going to lock you in your own house and force you to wait for her.
So what if she 'expects' you to stay in til she arrives. Who's making you stay in? Certainly not your MIL! It's starting to sound like you want to be a bit of a martyr over it all, to be honest.
It's really very (VERY) easy though. Text back if you must, but in very clear tones (why on earth did you waffle on before?) saying "If you come here we won't be in. I told you we have plans all day." And then GO OUT! Be out by the time her appointment at the hospital starts, and stay out. Turn your phone off/put in on silent, and if she calls or texts you, ignore her. It's really very simple, and not the agonising set of 'maybes and what ifs' you seem to be making it into.
No more wet lettuce, OP. If there's any comeback, you need your DH to support you, and you both need to be singing off the same hymn sheet. If he doesn't, well, as you saying goes you have a DH problem too! Good luck! 