Well I'm not sure what you are up against with the "fussy" part
But! I do have a small insight into GF and DF holiday meals. One of SIL's has Celiac's and at times she also needs to avoid dairy.
For holiday meals where she will be present I will do something like the following:
Appetizers = Veggie Tray with a "normal" dairy based dip and also a container of a yummy hummus flavor; Deviled Eggs are also dairy free - free of lactose anyway; A simple salad with choices of DF dressing (like Italian) and "regular" style dressing is also an easy choice
Main Course = Roasted Meat of some kind - Turkey, Ham or Pot Roast
Side Dishes = For her I make sure there is a DF Green Veg dish of some kind and Mashed Potatoes (for the potatoes, you can mash them with chicken stock instead of milk or cream OR use an unflavored unsweetened Cashew milk or a bit of both plus seasonings) I cook and mash the potatoes then divide some out for her. I add DF options to hers and the dairy deliciousness to the rest!
Bread = I use Pinterest to find DF and GF bread/roll recipes. The GF flours that can be swapped 1 for 1 with normal flour make things very easy nowadays!!!!! OR you can buy GF rolls at the store too!
SO much easier!
Dessert = There are tons of GF and DF dessert recipes. I highly recommend Swerve brand of sugar alternative. ESPECIALLY the Swerve Brown Sugar. That stuff is GOOD - I try not to use a lot of it all the time but it a really nice thing to use sometimes!
I've made the following recipe a couple of times - the blog author has a TON of info for making it GF, reducing the sugar, etc. I didn't have to worry about sugar when I made it so I can't speak to that part of it. BUT - as GF and DF cakes go - well I could have had the "regular" dessert but I ate two pieces of this!
https://www.texanerin.com/dairy-free-chocolate-cake/
Also agree that she may be more comfortable bringing her own food!