I think it seems rather sensible for them to stay put and spend the festivities with those they see and are in contact with the most, thus not bringing in more risks.
As to how funny or harmonious the festivities are in those households... I think that no matter what the situation, if Daisy was loath to spend time with the Derfs or Mary in particular, she certainly simply wouldn't - she could IMO speed secretly to Paris and have Xmas there, if the Schacks were so much more her favourites.
I think that Daisy is rather comfortable with the Derfs, and again: how harmonious will it be? Probably just as harmonious as in any other family with teenagers.
I agree that Daisy is hardly likely to choose to spend Christmas with the Derfs if she didn't want to. She strikes me as somebody who does exactly what she wants (within reason and often putting country first) she takes no fools and doesn't seem the sort of person to do something private because she feels she has to (public and official duties being a different thing) To be honest as much as I would question what the Derfs would let the kids get up to if Daisy wasn't there, as Fairy says, every family is like that to a degree. Our grandparents still expect us to dress up quite posh for Christmas day morning whereas when they aren't with us for the day our parents are quite happy for us to stay in pjs until dinner- I imagine the Derfs will be on best behaviour because that is what Marge wants and expects. That said if she wasn't there I imagine it would be a very laid back day for the family.