All of the above! I flip between Happy Yule/Solstice/Holidays to Merry Christmas pretty regularly, though I try to stick to Happy Holidays at least.
My aunt is seriously Catholic, and she wished me a Merry Christmas. I told her, I don't mind a "Merry Christmas" from my Christian friends, nor a Happy Hanukkah from my Jewish friens, or a Happy whatever from other religions as long as they don't mind a Happy Solstice from me.
As far as you dinner situation, you might remind them that you aren't Christian but still enjoy spending time with them on their holy day. Perhaps they wouldn't mind reciprocating for the same reason... (family guilt trip