They are 85, live in AZ but have been in Midwest for Summer. His side of family supports whatever he wants but my family sees our mother needs more structure as she deals with early signs of dementia. My daughter was going over 3 days a week to help until he essentially asked her to stop. He means well and loves our mother but his decision making is lacking. He is a terrible communicator and anytime the conversation turns to a subject that makes him uncomfortable he completely shuts down. The situation needs to change and I don't know how to do it in such a way that he can support it. They move back to AZ soon and no family is out there to help.
What about two smaller steps? Could it work to get all legal done for both of them, DPOA, MPOA, update wills, etc. for both of them that specifically names the family to be the point person for each. It would need to be done in the state that is their residence. So you might as well start to find good elder law attorney in Arizona for them. Now you also want to get a "Guardianship in case of Incapacity" statement done for them too. For your mom, it names you and another of your siblings as this. You are going to want this to use in the future so that you can file for guardianship of your mom and outside of stepdad's control. Also on the legal, you may want to pay for all this so that the attorney is "yours". He will be your point person in all things mom with you as his client as you are not in state. You may have to do a separate agreement to establish this and it will be priceless in the long run. Stepdad is only going to get more difficult over time. He too probably has dementia but mom's is more obvious. If he has a my way of the highway personality it probably hides the dementia better than someone who is sweet and more accepting of help (I bet that is your mom's personality). You have to do whatever now to be able to separate their needs and demands later on.
I do think Eyerishlass gets the crystal ball award for this month, she probably is right about what the future holds. Not pretty. So try to do what you can now.
What exactly do you and your siblings want, Mom in a nursing home, where you don't have to worry, but she will be miserable? That will eventually happen, please don't rush it. . You don't know the day in day out needs of your Mom's caregiving. You need to back off insisting your way is right when there are many other factors than safety to consider, such as happiness.
Do you know what your Mom's desires are?
It's a drastic step but I don't see this going anywhere good.
Now, back to discussing specific help. See, if granddaughter is coming and "hanging out," this may not be helpful to them. It could easily be viewed as being intrusive to their routine. (By him.) You don't say what your daughter was doing those three times a week, but I suspect it wasn't structured.
We all want to help. The trick is helping in a way a care giver wants us to help. That's value. Beyond that is not helpful; in fact, it can be irritating. Again, speaking as a care giver myself.
My mom's boyfriend comes over once a week like clockwork. He is a disruption in my routine, frankly. In addition to the myriad things I have to do for and with mom, I have to entertain HIM. Not fun. (Mom is much further along in her dementia than yours.) That's an example of a well-meaning intrusion.
Having said all that, you say they're returning to AZ shortly. Honestly? I'll bet he can't wait. Back to HIS structure, HIS routine. Do you kind of know what I mean? This is nothing against HIM. It's a reminder that dementia'd folks do better WITH routine and structure.
If you want to be helpful to mom/them, then maybe you could call Senior Services in their town/county and find out what might be available for them when they get back . . . how much it will cost . . . what it entails. Then have a conversation with him about taking advantage of those services in his own way.
Mom and her 86-year-old boyfriend lived together until a year ago when I took her into my home. In her own home, though, I set her up for someone to come in to clean her house twice a month (for two hours each time, so really only a lick-and-a-promise). That cost $28 a month. Then Meals on Wheels -- $5/day suggested donation. These services were thru the county. I'd imagine AZ has a plethora of senior services that might help your mom/stepdad. Trick is to find them and present them in a nonthreatening way that helps rather than interferes.
I understand your love and concern. Had mom been a thousand miles away from me, I'd have been beside myself.