I agree
Maybe start with the firm intention that Christmas with your children.. whatever day it "falls" on - is YOUR time with them. Don't get stuck in thinking everything has got to conform to some ideal or be perfect. That way you will find it easier to escape awkward conversations and stick to your plans.
Id' get my own presents for them, without reference to DH or his mum. Have some pre Christmas events with them. Book early (he won't) I think experiences stand out more than presents in their minds. f he or mil buy them the same things, they can either be kept at their houses or returned ( as you will have kept all your receipts) and you can buy something better in the Jan sales.
I wouldn't give Ex and MIL any clues whatsoever about what to buy them, as (if they are anything like my relatives, they will just nick the ideas and then I'd be left scouting around to find something else as good) just say "ask the DC, they are full of ideas"
He's demonstrated that he's a man who values his free time pursuits and his money over everything else, you can't change that.. so don't worry about what he does or doesn't do. let him get on with it.
But you already know they will have a lovely Christmas with you, so what he does, doesn't matter. They will still probably enjoy themselves and MIL will try hard for them I think. They won't be miserable.
But you will find conversations springing up about Christmas, maybe be cheery and slightly evasive with MIL without being outright unco-operative but also without giving away any information that might get passed to DH. But I'd have some extremely evasive answers for DH, he will only fault find or want to insist on the opposite. Let him find fault with his own arrangements.
You will have a lovely Christmas this year @mummy917 with your little ones because you won't have all the Twatoo nonsense ruining it, and will be looking forward to better times ahead.