What does your DH say/do to all of this? It's hard to say stuff to your in-laws and I get that you don't want to offend them, but if he isn't helping or supporting you, then why?
commenting throughout the day that the dc have far too many presents and being sulky when the dc play rather than making small-talk (they are 4 &6 and very polite, just likely to spend more time playing than chatting.)
"Yes, they've been very lucky this year, haven't they? DC1 has been wanting that xxx for ages, it's lovely to see how he/she is enjoying it, and don't the DC play together well? Now, can I get you another drink? Is there anything you'd like to watch on the TV - there's a great film on soon/the Queen/horse-racing (whatever!)."
ignoring the way the table has been laid and pushing decorations and settings aside to make way for the carrier bags of crackers, humous, etc that sil tends to bring.
I'm not sure I'd be that bothered about this - at least they are bringing you things - but as it bothers you, I think I'd just collect it all up and put it on the side/in the kitchen/out of the way without saying anything. If they comment, you can say something like "I need the room on the table to serve the food/put out the wine glasses" I can't really see the problem with crackers though - do they really spoil your table/decorations?
giving the dc their present while I am in the kitchen, so I don't get to see it opened, or know exactly who has given what
This is awful! I think I'd be insisting that all presents go under the tree, from when they arrive. And then present-giving all takes place at the same time, for everyone. I'd be pre-empting this, by saying "OK, this is what we're going to do tomorrow. The DC will have their stockings first (if this is what you do of course!), then we'll come down for breakfast and after that, we'll open all our presents" Or whatever your preferred routine is - but you spell it out and make it absolutely clear when presents are being given/opened. And then you get your DH to object very loudly if they try and do any of this when you're not there.
Mil walking around the house and garden and commenting on everything that needs doing, how expensive it will be, how the house will fall down if it's not done immediately, how we may have taken on too much etc etc
Nothing you can really do here but agree
There's no conversation to be had. Just ignore her.
Oh, and gin. Lots of gin. (For you, that is.)