At your final fitting you should bring a close friend or relative with you to learn how to get you in and out of your dress as well as how to bustle your train. This person should not be your mom. Have them practice several times to ensure that they know how to do it, especially if your dress has more than just a zipper.
As for the bathroom, I’ve scene this scenario several times. In the event that you can’t use the restroom on your own bring the person who knows your dress with you. Think of her as your bathroom buddy.
She will either get you out of the dress in time to use the loo or hold up your skirt if need be. Remember this is supposed to be one of your closest friends.
And the reason I don’t suggest your mom is she is probably in a very elegant dress and it is not wise for her to be getting on her hands and knees to bustle your train. Though if she’ll just have to hold your skirt I’m sure she’d be honored to be your bathroom buddy.