Often when older people start to noticeably lose their memory, routine becomes a necessity. It is the only way they get through the day while trying to hide the fact that they don't remember even what day it is (does she have multiple calenders throughout the house?). The crankiness could be part of that fear, or it could be part of a personality change associated with Alzheimer's.
Here's the rub. Assisted living will not take a person who is already diagnosed with Alzheimer's. Once that diagnosis is made, it is ether home care or a nursing home. But older people, in general, do better in a assisted living as they are surrounded by functioning people their own age with similar interests. But if she is in the early stages of Alzheimer's, there are an amazing array of new treatments that assist in slowing it's progression (coconut oil is said to help when added to other treatments).
I'm sorry, this is a difficult situation for anybody. I would suggest that she get her financial and legal paperwork in order while she is still able to make decisions. Will, legal power of attorney, medical power of attorney, and an advanced health care directive at the minimum.