Scout, the thought of Christmas without my children makes me want to vomit, so I totally understand where you are coming from. 3 out of the 4 are 'believers' and thanks to Covid and not being in person in school, I anticipate it will stay that way for this year. But I too feel the magic will only last so long and am so sad to have to lose even a moment of that stage.
But I also think that in some ways it may not be the hill to die on. DnJ is right in that plenty of families have to be creative on their Christmas 'timing' and really, in the end, the children benefit by the extra love and attention from multiple families. Cold comfort right now, for us both.
2x4? The thing about Christmas is that the whole family will have a strong opinion with their bias interfering. So it will be H's whole family against you as they know they will 'win' or 'lose' in this particular fight too. Do you really want to take that on for a few hours of Christmas morning magic? Do you really want MIL picking apart the parenting plan for details they could use as negotiating ammo for Christmas custody exchange?