I am sorry, that is a very difficult situation. I don't have a lot of experience in this. I have a few ideas:

Could you directly ask your father if your mother is too much for him to handle? Perhaps he wants help buying the house near you or finding a senior living arrangement?

If that doesn't go well or isn't a possibility, I would look for agencies in your parents state that deal with aging. Maybe DHS. An Alzheimer's support group or advocacy group. Tell them that you think your father can't handle your mother anymore and ask them to evaluate. Someone must do that, right? Could you call your mother's doctor and ask them to recommend a visit from a nursing agency?

Alzheimer's can do some really strange things to people. I just watched a neighbor's husband live his last year with early onset Alzheimers and he became a different person. He remembered quite a bit, forgot a lot, but his personality seemed to change sometimes. Maybe your mother is being abused, maybe she isn't. I think you are right that you are not going to know for sure based on what she says.