My husband says my MIL, who lives in FL, will probably join us for one day in the theme parks. We will be going for five days. He can't give me specifics on which day and I think it will likely be just whenever she can get away/feels like it, etc., and won't be decided until week of the trip. 
Of course, I've got dining reservations and FP reservations set up already. There's no exception that could be made by getting her a carbon of our FPs day of, is there? I think I know the answer, but I'm hoping against hope.
I'll buy her a one day ticket and add her to our resort stay so she can stay with us, but that one day ticket will only allow me to just pick one random day to schedule her FP+ right?
The other thing I could do is buy her a whole five day pass and then just make her FP with us. Of course, four days will go to waste and that is a lot of money.
Any suggestions?
I am not close with my mother in law and although we are in a good place right now, I'm afraid that my husband being all chill with her coming up for a day and me being like, "What day will you be here? How long will you stay? Would you like to be included in our dining reservation?" will be taken as me not wanting her to come, which isn't the case. It's just that I'm trying to figure this out on my own and it's not a case where I could just call her and explain things straight up.

Of course, I've got dining reservations and FP reservations set up already. There's no exception that could be made by getting her a carbon of our FPs day of, is there? I think I know the answer, but I'm hoping against hope.
I'll buy her a one day ticket and add her to our resort stay so she can stay with us, but that one day ticket will only allow me to just pick one random day to schedule her FP+ right?
The other thing I could do is buy her a whole five day pass and then just make her FP with us. Of course, four days will go to waste and that is a lot of money.

I am not close with my mother in law and although we are in a good place right now, I'm afraid that my husband being all chill with her coming up for a day and me being like, "What day will you be here? How long will you stay? Would you like to be included in our dining reservation?" will be taken as me not wanting her to come, which isn't the case. It's just that I'm trying to figure this out on my own and it's not a case where I could just call her and explain things straight up.