Fortunately, I have not had to deal with Alzheimers, but there was one realization I came to that comforted ME and helped me better deal with my dad as he suffered from spells of dementia in his declining year.
I would stop by his assisted living apartment each evening on the way home from work. Often he would tell me about his day "in the woods" - he had been a forester. At first I would try to correct him and tell him he had really been here in his apartment all day and not in the woods -- but the next day, he had been working in the woods again.
One day God broke through my dense skull - "I am with him wherever he goes."
Then, I knew he was in good hands, and I didn't try to correct him any more.
Seeing him like this was more difficult than I ever expected, and I can imagine how much more terrible Alzheimers is. We don't know the road God has planned for us, but we can, beforehand, KNOW that He will never leave us and never forsake us (Heb 13:5). That is a comfort for us as well as our families.