Try to think of Christmas dinner as a glorified Sunday roast, which gets it in perspective. Decide on your menu, but don't do over complicated things and make more work for yourself than necessary. Now is not the time to make recipes you've not tried before or aren't confident with. Have a trial run one weekend with a chicken or something. Make sure you can accommodate all the things you plan to cook in your cooker, and that your turkey will fit, giving enough room for other things as well. Also that you have enough pans that are big enough. Sort out enough crockery, cutlery, glasses, serving dishes, chairs.
Prepare things in advance, write up a cooking schedule so that you can stick to it, and you won't be stressed. If MIL wants to bring some food items, just serve them alongside what you have planned. Don't see it as 'giving in'. Of course, if you genuinely have no room in the fridge to store them, you will have to tell her so, and "it would be such a shame if they were spoiled because the fridge is full, and we can't keep them chilled." But extra baked items would always be welcome, and guests could take some home with them if there's a lot left.
Definitely give specific tasks to do, what about setting/decorating the table, preparing veg on Christmas eve, playing with DC if you have them yet, making morning coffee for everyone. Get her to help carry food to the table and clear away plates, etc.
I know you want to prove yourself, but every head chef has a team of staff behind them, and your guests will know that it wasn't all your mum's doing! I'm sure the rest of the family are used to her by now, perhaps one of them would be happy to distract her for you for a while, when it's clear she is getting on yours and DH'S NERVES? (Or ply her with drink.)