I think it is completely unkind and unnecessary for people to call you cruel.
How judgemental!.
If you already have 10 people to cater for on Christmas Day, the prospect of suddenly having more people, ones that you don't know well, coming on Christmas Day evening, when you've already planned all your shopping etc is likely to throw anyone a bit.
That said. It is just an evening and your DH clearly feels he wants to do that, I'd be inclined to let him. When someone dies people are often wanting to do something, even though they know it won't change the fact that everyone is in mourning, they just want to feel they've done something.
So could you get DH to make a dash to the shops and buy some buffet food? so you don't need to cook more. You will have Xmas leftovers so You probably won't need all that much for an evening. So that you don't have to do too much extra prep. Then he can entertain them and you can give them all a bit of space ( and take a breather yourself). The post Christmas clear up shouldn't take too long if everyone mucks in. And I'm guessing they won't be bothered by the state of the house, etc.. they will just want to be together and be sociable for a bit.
I think this would be a better solution for you in the long run, and would be nice for your DH to feel he's made an offer. They may not even accept for that evening or maybe prefer another day over the season.
I hope you work this out, you sound quite overloaded but you've still got a few days and get DH to help.