This is a tough situation and you sound like someone who is pretty upfront about things and not afraid to be direct when necessary. I don't think there is 1 right answer but you and your husband should list out different options and talk about the pros/cons.
Questions that arise for me just from the limited info you have given.
Are the problems with the ex really going to be solved by moving closer or is it going to only get worse?
If you are not there to care for grandma, then what care options will be put in place? It is unlikely she will be able to stay in her home long with the age and Alz factor. Does she have $ for in home care/assistance? Can you find trusted help for her? Will she tolerate it? Many Alz patients can't handle having a "stranger" in their house.
How does moving now affect your family financially?
In terms of the holidays, discuss options with other family members. Either you need the holidays to be celebrated elsewhere (time for the torch to be passed) or you need financial assistance to hire someone to come in a clean grandma's house since your plate is too full.
Good luck to you and huge holiday hug.