Definitely a difficult situation for you. I agree with the advice to discuss this as little as possible and be very mindful of your body language and tone of voice. (My mother can't always understand what we say, but she is frighteningly adept at picking up non verbal cues.). Of course this is easier said than done, especially if there are preparations to be made which differ from the usual routine.
I also like the idea of dismissing this as a routine test, very minor, nothing to worry about-whether it is or not.
From a practical standpoint, is there anyone else who could accompany you, so you have an extra pair of hands?
The last time my mother was in hospital, they were very good about allowing me to accompany her for tests and things, even places where family and visitors are not normally allowed. It can't hurt to ask.
I also had a note in my pocket explaining that she has dementia, that I'm her daughter and PoA, that she can't answer questions, and to speak to me out of her sight and hearing. I just handed that to everyone I encountered, and staff were very respectful and understanding.
I also wonder about going somewhere after the procedure, someplace pleasant or enjoyable, or for ice cream or cake and coffee or some other favored treat, to have a pleasant memory and feeling about the day, not just hospital and ick.
I hope all goes as well as possible and best wishes.