It seems like there is more than one dimension to this. Not clear why arrived five days early. But typically if you've been invited to someone's home, it's not required, expected or even really polite, that you would take necessary actions and facilitate the event. Unless they flagged they want you to do that stuff, or they figured you would do it, in which case their motives are suss.
It's sad if they have not been looking after the jointly-owned home. But that's a separate issue from Christmas. You've said your kids are having a great time, and it's totally normal for kids not to notice what's going on among the adults. Therefore, I don't think the answer is to go home.
I would actually just say, "okay, I think I've done enough to help prepare, at this point."
And relax for the rest of the time. If they're not prepared for Christmas, it's on them.
Because your kids are there, I'd just focus on meeting the bare minimum for them. The kids won't care about the food, decorations etc. - only their presents. Do the minimum if you can't avoid it, but don't do any more because clearly you resent it.