I have more experience than you think. I have seen both problems come on slowly in my times working at nursing homes and serving as a Pastor.
I cannot medically diagnose them, but there are signs.
Yes they remember things, some times, but they have those moments where they are clueless.
As time goes on, they are completely robots doing what they are physically led to do.
A woman of 73 at this home I worked at, has alzheimers and she has to be led around and do as she was told as if she is completely controlled by the care giver.
She eats when told, she walks around aimlessly, she will even be allowed one cigarette a day, because she always smoked. She will take a few drags then put it down and forget about it. They do what they can to make her life seem normal, but in fact she is not there mentally.
She got that way slowly, but you could tell when it started, and knew where it was leading.
I had an Alzheimers patient who was led into the sermon, and she sat their staring an looking around, but once the songs began, she sang the song word for word and in perfect harmony as if she was 20 years old. Once the songs stopped she gone again.
Re: your last paragraph.
I used to refer to them as doll eyes. They look at you with recognition, you talk, and then they go “doll eyes.”
When my mom did that, her 55 year old son (me) knew it was almost over. I went to the parking lot and cried. She was gone after that.
I appreciate your experience “creds”. I don’t agree with everything...but what a boring world THAT would be. Have a good Sunday, pastor.