Is there anything I can do?

[DVCkidsMOM]

I would have the same situation with not being able to ask MIL what day, what dining, what attractions... LSS, those questions right now would be seen by MIL as demands or an attempt to control if I asked them. The only way I could get that information in a way that would not be threatening to her would be to have DH ask those questions. I would really need DH on-board to ensure that he would enforce reasonable boundaries and expectations for a last-minute addition of his mother to our touring plan.
I don't think I would recommend making all ADRs to include an extra maybe. Sometimes you can call and add a person or two to an ADR - I think I would consider that before I would have a larger table reserved everywhere knowing I might need it only twice. We called to add people to a January 2014 ADR at Cape May Cafe with success. I'm sure not all locations can do this all the time, but it can happen... I would do all I could to get DH on the same page that I was on about what we are willing to try to change and what we are not willing to change, as well as what we will do if something we are willing to change can't be accommodated by WDW. I would ask DH how he wants to handle his mother's last minute travel. She can make her own FP+ for about the same time as yours if she wants to partake or, if she asks nicely and truly understands that try does not equate with success, DH can link her pass to his and try to copy FP+ whenever she determines a travel date.
If the situation is as you say and you can't ask MIL the exactly when and what questions, then the only way to get through this without arguments and resentments is to put DH in charge of "adding" his mother to the plan. You also have to make sure DH understands your priorities and requirements so he can ensure that his adding his mother doesn't annihilate your travel joy. ...just sayin'...

 

[chloelovesdisney]

The rider switch passes are good till the last day of the calendar month. You don't need to get them in advance though since you will really need someone to stay with your toddler if he/she doesn't meet the height requirements anyway.

I wouldn't worry too much about it, you're only talking about 3 attractions over the course of the day. She may not want to go on all of them anyway, or your husband can give her his magic band and she can use it in his place. He'll miss out on the ride, but it's his mother not making advance arrangements.

 

[leebee]

To cut the MIL some slack, I believe the OP said she can't make her FP+ reservations until January. That means their trip isn't for about 4 months. I can see how the MIL might not understand which specific day, 4 months from now, she needs to commit to going to Disney. OP, ask her again at the 30 day point and see if she has a better idea of what her schedule is going to be.

 

[AC7179]

We will be going in March.

Right, I don't think it's my MIL's fault, as I'm sure she has no clue as to the way Disney works. She's been to the parks before but I'm sure doesn't do anything to prepare her before she goes. So it's not something she's doing on purpose, it's just that I don't have the kind of relationship where she would just follow along blindly while I plan everything OR where we could plan it out together. We will just go with the flow that day and make it work the best we can!

 

[Mambo Junkie]

This is how we handled it. I had 2 late arrivals to our trip, my SILs. I called and the only ADRs I could not add them to is CRT and BOG. I was able to add them to: Coral Reef, CP, 1900 PF, and Tomorrowland Terrace Dessert Party. I did not try H&V lunch, but got them a 50s Cafe at the same time. I WAS able to get them a BOG while we did CRT and a CRT while we did BOG, lol. I have been able to add them to every fp except 7DMT but I decided not to ride it so my 1 SIL who likes that sort of thing is taking my place while the other SIL and I go meet Cinderella and Rapunzel with the little ones. We did not worry about the M&Gs but got them something they would enjoy at the same time. MOST of the fps I was able to at least get an overlapping time. A couple of times I just dumped our whole day of fps and was able to book us all as a group.
You can work around it. Oh, and I could not (try as I might) add them to our LTT ADR, but the CM suggested the Diamond Horseshoe Saloon, which I am thrilled about. We ate there in 1972, but ever since then it has been closed for the season. Since I have been every season there is, I wasn't sure if Disney had a secret 5th season or not.
Good luck! It'll work out; don't stress over it!

 

[BillSears]

I had a similar situation with an Uncle joining us. I decided I'd send the rest of our party with him using my FP+ and I'll just hang out while they ride.