SuzQ... I remember you well and I am so very sorry you are having to deal with this again. Know that many families are hit multiple times. I have had 5 different trips though this disease just in my immediate family, not to mention the aunts and uncles. My grandmother had Alzheimer's, my father had Vascular dementia, my FIL had stroke related (probably Vascular) dementia, my MIL had undiagnosed dementia (can you tell the in laws were in denial?) and my mother has Alzheimer's. I am so done with this disease!
I do believe that you are right in what you have said in your post. Anyone with dementia does need stability and a routine. It is difficult for them to "change gears". Each family has a different routine and it is a new environment for her to adjust to. Doing this over and over can cause unnecessary confusion. As the disease progresses it will only get worse.
As for the trip, you have to make a clear assessment of her abilities and how she adjust to new and different routines. You have to know that dementia patients do not deal well with crowds, noises, and unfamiliar situations. She will probably not be patient standing in line or waiting for events (even meals). If she has any sundowning then your evening hours will need to be planned to accommodate this. She will have a tendency to wander away and has to be watched closely 24/7 while you are there. Know that she is not going to "go with the flow" of the family but is going to be on her own trip and you will have no choice but to follow her lead. Be prepared for her melt downs when it all become over whelming.
Some think it is worth that last trip to include Mom. If you take it slow, follow her lead, and are willing to alter your vacation to accommodate her needs, it might work.... depending on her abilities and inabilities. But if the family plans to keep up their pace, go go go, and think Mom will just keep up, they will be very disappointed.
I do know some that have tried this and been successful, and I know some that have tried this and ended up coming home early because it was a disaster. It all depends on where Mom is in the disease and how willing the family is to accommodate her. So you have to have a conversation, as a family, to determine if the desire to include Mom is logical and rational, or just wishful thinking