Fluctuating mentation. Is that common or specific to a given stage?

Yes I believe so. I've been wondering the same thing. My hubby was diagnosed with early onset alzheimers. I've looked for answers to this same thing...I felt he improved because it's not really his memory, but came to realize with him its cognitive loss. Which I will say has helped me to be able to look for different things and stages. Every article I've read has said it's a roller coaster ride mixed in with landings along the way. Slow and steady then whoosh.

Yes, one day dad would do real well and the next totally different. It could even change in a span of hours. Mornings after breakfast were usually his best times near the end.

I think that is true. I cared for, then lived with, a couple a little over five years. They both had dementia but expressed differently. Doctors say there is no medical cure, only medication to slow it down. The Mr. was repetitive and forgetful, which progressively got worse, but he had pat answers to routine questions that made it seem like his affliction was mild, but it progressively was more severe and noticeable. Whereas the Mrs. would have the nonsensical talk, be non-communicative, call out for people who long passed away, occasional hallucinate, or be sharp as a tack and still is.

From my personal experience, it is a rollercoaster ride.

You never know who will wake up tomorrow, loving gentle g'ma or satan on steroids. It is not easy and when we come to terms with the reality that there is no reality for them it helps us cope better.

Hugs!

Alzheimer's typically is slow and steady. Some slower than other but the pace is pretty steady.
My Husband was "diagnosed" with Alzheimer's but later I was thinking that he also had Vascular Dementia. That typically will have steep declines. I said Alzheimer's is like walking down a ramp, slow, steady decline while Vascular is more like stairs with landings between the declines where things will hold steady for a while. The two combined was like a ramp with a few stairs thrown in every few paces. I never got an "actual" diagnosis because by the time I learned enough he was past the point where it would matter, no medication would help and knowing would not change the outcome and there was no reason to subject him to tests that would not help.
What does help is a set schedule, being consistent with getting up in the morning, shower routine, breakfast and everything else that goes on.
If your LO is a candidate for Adult Day Care take advantage of that. Socialization is good and if this is part of the routine even better. And it helps you get a break. And decide at what point you will consider a Memory Care facility. Get all the papers you will need in order POA for health, finance, DNR or a better document is a POLST. there is a document you can get on line called Five Wishes that will help a lot with planning.

From what I understand true Alzheimer's is a slow and steady decline. Other forms of dementia, like Vascular caused by strokes will drop suddenly due to a stroke and then plateau until the next stroke. I believe my dad has vascular as he has had hundreds (probably) of undetectable strokes and some that do have obvious symptoms. In Sept for example he had a stroke during the night (we suspect). He woke up with no knowledge that his wife, sons and his mother and siblings were all deceased. Now every day, many times a day he asks about them. He "seemed" to improve somewhat but not totally back to his previous person. You can read about the different types of dementia (which is the umbrella term) at the Alzheimer's Assoc website. Hope this helps.