mrsvetinari Lots of good advice there from OPs!
Having read through all the posts and threads up to here, I have noticed that many of the women on here have joined the The MK Cult, initially, for their own sanity! 
However, as they have whirled round kondoing their stuff and communal stuff, a lot of DPs have seen the light - to a greater (sheds, garages and DVDs cleared!) or lesser extent (cack-handed folding, or books and komono gradually disappearing) and have sneaked into the project. It is infectious!
Children have, generally, been enthusiastic, but maybe that was in the expectation of topping-up again over Christmas! 
If you do most of the housework, your best argument about the ornaments, could be that they interfere with cleaning and are (as CoolCadbury said) unnecessary clutter.
Involve your DP in deciding the best way to dispose of unwanted stuff - selling, donating, recycling or the tip. Perhaps he would take on the job of selling stuff on eBay or Gumtree?
Once he starts to see how much tidier and happier the household is, with less stuff and more clear space, you may well find him more supportive - even motivated!
The photo idea is good. Wear the bleh necklace and catch an apparently unposed snap of the family, for the ILs. Then send it to the charity shop.
Best wishes.