Mom has Alzheimers dementia; has had for 8-9 years. Do Alzheimers patients develop a sort of 'plateau' of calmness and coherency?

Alzheimer's is slow and steady although there can be some fast declines at times. A lot of the fast declines could be attributed to an illness. A cold, medical treatment, an intestinal upset, possibly even an unrelated family emergency where there is extra stress within people.
My Husband was "diagnosed" with Alzheimer's but after a while and reading I think he also had Vascular Dementia as his decline would be slow and steady then it was like he would level off then all of a sudden he would have a pretty significant decline then he would even out again.
It is possible to have both Alzheimer's and Vascular Dementia. (a Neuropsychologist I heard speak said that the combination was like "winning" the lottery of dementia's...and my Husband never did anything half way so if any one could have had both it would have been him!)

Bottom line..
Her dementia will get worse, she will decline. If it happens suddenly or bit by bit it does not matter because nothing can prevent what is. The damage is done nothing will ever change that. Even if they came up with a "cure" would you want the progression to stop now?
Accept the plateaus she has, be grateful for the breaks in decline but also accept it will continue.
((Hugs))

I agree with all of the answers you have above, especially Grandma 1954 who points out that it is possible to have both Alzheimer's and Vascular Dementia. Here in the United Kingdom that is called "Mixed Dementia" and is well known.
That is the case with my wife, also some eight to nine years into Alzheimer's, who appears to have reached a plateau over the last six months.

Each person with dementia is different, but there does appear to be a key decision that people with dementia face: stay well as long as possible or let yourself go downhill fast. This is set out clearly in June Andrews, "Dementia the One Stop Guide: Practical advice for families, professionals, and people living with dementia and Alzheimer's disease (a key chart is on p. 38 and repeated often throughout the book).

As the dementia journey continues, the person (and the carers) going on that journey both change. Dr Jenifer Bute with Louise Morse points out in "Dementia from the Inside: A Doctor’s Personal Journey of Hope" that: (1) There is always a reason why a person is behaving in a particular way; (2) When facts are forgotten, feelings remain; and (3) Familiar patterns of behaviour continue.

Nicci Gerrard’s "What Dementia Teaches Us about Love (Allen Lane/Penguin, 2019), is a brutally honest attempt “to make sense of an illness that has the power to dismantle the self, that comes like a robber in the night to sneak into a house built up over a lifetime” (p.2). A key theme is that “the diagnosis [of dementia] is not a sentence but the start of a process that can take years, even decades, and that can contain hope and kindness and adventure as well as fear and sorrow and heart-wrenching loss” (p. 9).

Reading these three books could save you a lot of trouble and be helpful in trying to understand what is happening to your loved one. Nicci Gerrard sets out the pitfalls, Jennifer Bute the hopeful possibilities, and June Andrews a balanced reflection on how dementia might develop after a diagnosis.

With my best wishes and prayers

I’m not sure there are “plateaus” with this disease, or if we, as their caregivers and loved ones, just become accustomed to their behaviors and adjust accordingly. But I do know there is no “reverse gear” on this disease and it sadly just gets worse.

My experience was they plateau, then decline, plateau decline. Various areas of the brain are being affected, so that's why you can observe what you are.

You will read that this does happen, and it might, but it did not happen with my husband and he was also on several medications like the ones you mentioned. Things to look out for are urinary infections. They are often hard to figure out, but sudden changes in behavior are a clue. Better to have it checked than allow one to go undiagnosed. Also, coughing and choking on food can be a sign that the loved one has entered into a new stage. Thus is very dangerous as food can be aspirated into the lungs, causing pneumonia. That was the beginning of a rapid decline. My husband had a "swallow exam" and was placed on all thickened liquids and pureed foods. I was able to purchas many good food products online, but tried to create them at home from scratch too. I hope your experience is one that has this "plateau". My husband had frontal/temporal lobe vascular dementia. His last three months were so very sad.

You've received some helpful answers, particularly the one with references to books which I will note myself...but clearly they are questions you can ask of the MD prescribing the meds. For instance there may be a point that you might want to consider stopping them if there is no obvious benefit. My attitude is to just take it one day at a time because you never know what the next will bring and I am not going to waste time with all this BS about stages and determining which one is in. What difference does it really make? You're still a caregiver dealing with what the day brings. Not that one can't plan ahead for things being worse...but I won't dwell on it.

I have no experience with Alzheimer's, but I wonder if at least part of what the OP is seeing is that along the spiral downward there may be points at which the patient loses the mental ability necessary to become agitated. I recall reading where others have said that a certain stage "will pass" and the patient will become easier to manage. Perhaps if the capacity for becoming agitated is lessened but the cognitive skills don't decrease accordingly, what is left of the latter can be put to better use due to lack of interference from what remains of the former.

My mom is at the very end stage of Alzheimer's disease (AD), having suffered symptomatic AD for 10 years now, and I do not expect her to live beyond a few months at most. With her profound terminal illness she *is* calm and there are times she actually says things to me; even some occasions will answer appropriately with short answers. It kind of shocks me when that happens, but it does. She is not improving, but at the end of her terminal illness and there is nothing I can do about it but care for her the best I can. She no longer impulsively gets up because she is too weak to do so. I noticed every time she declines, she does so quickly. She will be at this baseline for a long long time, then suddenly drop to a new level of disability requiring more intense care. This becomes her new baseline..then something else happens like an illness or just time then drops to another new level of weakness and disability. She has been doing this for 10 years now.

What I have experienced with my father who has Alzheimer's is a roller coaster. Sometimes it goes slow, sometimes you think it's stopped then you pick up speed and have a hard decline. And the reality of severity hits you, and you have to take a deep breath and not react. My Dad thinks he's just naturally aging. He disputes the diagnoses despite "profound deterioration" found on his MRI. He admits he's having memory issues, but literally yesterday, he wanted to know if JFK was still around. Today, he couldn't remember what the name of our town is.
Tomorrow, he might hold a conversation for several minutes where he's completely clear and aware. Yes, I do believe there are plateau's and they change constantly. It makes trying to get an idea where the illness is, how far has it progressed and what is still ahead, very difficult. Only solution I can offer is just love your Mom, remember the saying, "Love your family, they are on loan to us by the Lord. One day he'll call in his loan." Hang in there!

Like Gloria said. It's a roller coaster. We just take it day to day, one interaction at a time. In five minutes, it'll all be different anyway. We just try to keep her happy in the moment. And go from there. We don't look at the future because we have no idea what that holds anyway.