My mother went through Dementia. I highly doubt it was a fake. She would have moments of absolute clarity then be off into the ozone a few minutes later. With enough drugs, a level of clarity can be achieved, but the decay does not stop.
Ever notice that Joey’s moments of clarity don’t last as long as say 3-4 months ago?
We saw this with a loved one as well. Good hours and bad hours. Late in the day was worst (sundowning) and taking a walk or increasing hydration helped a bit. He seemed to have a mix of Alzheimer's and vascular dementia. I always figured that when we raised his hydration and blood pressure, a little more oxygen got through to his brain. By evening, his brain was just plain tired from fighting to function.
Drugs didn't help much. He had a succession of mini strokes and falls, and each time he had a step-down in function. I was really sad for him when this caused him to be fearful of his surroundings. He thought there were rats in the bathroom (so he was scared to use it) and that revolutionary organizers were plotting in the hallway of his nursing home. Drugs to calm him just made him a zombie. No easy answers.