Diane,
You are such a wonderful daughter to be helping your mother out the way you are!
I don't have experience like yours, but may I suggest a few ideas.
First, does your mother have the same GP. If so maybe make the excuse of her going to the GP and "since it's so far" he could also see him at the same time.
I am assuming that he doesn't have the same GP. If he truly cares for your mother she could try a "good day" to talk to him to consent. If he has others whom he knows to accuse him of Alzheimers or dementia straight out and he is straight forward that he does not, use this as an excuse to prove others wrong. If he is the type to be competetive he may be willing to do a mmse to prove evryone wrong. If he passes then your mother will have an uphill battle if he doesn't then she will know a little about where she stands. But by all means make sure it has been documented and given to his GP as to his behaviour.
Sometimes an AD patient becomes anxious over the change they see in themself (I know not a good grammatical choice). They take it out on the one they love the most, because they are closest. There is no excuse..except for the disease itself. I'm not condoning abuse of any sort. I am just trying to relay from their point. The best way to handle a situation is to put yourself in their shoes. Understand that their memories are leaving them and as a normal person we forget time from the longest of time ago and as an AD patient most of them forget memories from their short term. (Now how nerve racking is it when you lose your keys?) I am not chastising.
I have a dr. for mom who is "older than time itself", by his own standards. He teaches humanities at a college to budding physicians. The above paragraph is how he explains most patients of AD. He says that is because they are taught to be dependant and giving up dependance is hard. It takes away pride. Mom was not taught to be dependant on herself when she was young so when she became afflicted with AD she became like she was as a child..very submissive, not wanting to harm, hurt, get in the way or anything. She would sit very still and quiet for hours. I HAD to litterally gently force her to talk and help out as she would be scared of doing things.
Perhaps letting him think he is in control by gently manipulating the situation to his ideas is what I am saying. In the meantime support your mother, if he is capable of taking care of himself for an hour or two maybe you could take her out for quality time. I'm sure just to pass the time with her daughter would help her and if she wants to talk about him let her, if not I'm sure there are a milllion other things to talk about that will mentally get her out of her "problem" right now.
I would also recommend that when it comes to him choose the battles that you will fight carefully. There will be a ton of battles not worth the time and frustration for your grandmother, her partner or you. Let him do as he pleases (within means..no hitting or hurting others) Write down the situation in a diary. After time your mother may start to see a pattern. (Tired, hungry,just found out he messed up on something). The diary will be a good back up for her and you, it will give you a way of seeing his whole like in the big picture.
I can say I know people in here have better ideas and have had the strain of an abusive AD patient. You may want to read thru some of the threaads and find those then look specifically to other posts by them...they may lead to valuable knowledge.
As for your mother feeling guilty, that is only her human nature, but you can ease her mind in knowing that if that is what it comes to she is still being compassionate. There are lots of other people who have put their loved ones in homes. Some because they too care and love their partner but it is just too much for someone at that age. (I didn't mean it the wrong way...caring for an AD patient at home is hard..and the later stages are really hard and their comes a time when you must think of yourself as so many times when you don't think of yourself the carer dies first and then who takes care of the patient?) I too seriously recommend an AD support group for her, but perhaps, if she will not go, and has access to a computer she could join you here on TP. I think if she saw what others are going thru it may help her too.
I really need to learn how not to be so verbose, I apologize for the long post...I also apologize if I said anything in such a way that I stepped on toes...not trying to. I do hope you find some answers to ease the mind of your mother and make it easier.
Nancy