YANBU. People aren't being sensible with their replies. They sound like the sort of people who can't comprehend how small your home is.
If it's so limited in space that the dining table is shoved into an alcove then there's no room for a second table next to it, especially once you've pulled the dining table out so you could all sit at it. Due to this you also wouldn't even own a second table, even a folding one, because you'd have no possible use for it and nowhere to store it.
Of course an extra person matters when you already have 6 squashed around a 4 seater table! Trying to squeeze in a 7th person is not really going to work. It sounds like with the table pulled out of the alcove it just doesn't work anyway because nobody can easily move around it. Hence your decision to do a buffet. I totally understand that it's not ideal or traditional, but if you've got 6 people in a tiny home it's the best you can do and still have ease of movement around the room.
Your DS is trying to pretend he's something he isn't to get the girl. That's a no. People really need to stop doing this. If everyone was themselves from the beginning, people could more easily and quickly work out whether they were compatible. This notion of being on best behaviour in a new relationship is so ridiculous because it can't be kept up long term. All that happens is someone falls in love with the fantasy version then, when the behaviour slips and they revert more and more to default personality, their partner becomes disillusioned and resentful until things get so bad they break up. Who is that serving? Nobody. Not in the long term.
So she's from a posh family, so what? He isn't and there's not even enough space for a traditional roast Christmas dinner in his family home. Either he earns enough to eat out Christmas day or he doesn't. He is what he is and he's from where he's from. She needs to either accept him as such, or not. Expecting him to be something he's not, is not the way forward into a happy lasting relationship.
On the other hand if this isn't a serious relationship and he's just milking it for what he can get out of it for as long as it lasts, then he can do so on his own dime and at his own convenience, not yours.