It's still too soon to say Merry Christmas unless you preface it with "If I don't speak to you again before the holidays..." That's the official rule at this point. You can't start the early blessings any time before December though even if you don't expect to see the person until after Christmas. So, don't wish anyone Merry Christmas in July even though you don't expect to see them until January.
Right now, Happy Holidays is very appropriate in just about any setting. I reckon Advent is a holiday and celebrate the Advent season. I'm also big on the Epiphany. And Yule. And the Winter Solstice. So, there are many holidays for which blessings are proper. And why shouldn't I have a Happy Hannukah just because I'm not Jewish? Don't I deserve a prosperous and joyful Kwaanza?
If you don't really mean it, keep your blessing. Or give whatever blessing you really feel like giving. It's the same rule as applies to everyday blessings. If you're not up to an entire Good Day, then give a Good Afternoon or even a Nice Late Morning. Don't give out too big of a blessing, either. Never wish anyone Merry Christmases Forever and Ever because it will be awkward when you see them in subsequent years and have to remind them that you already gave them a lifetime supply of Christmas blessings.