Didn't read much in this thread, but being deaf, I'll point out that it is important to us that we are called deaf or hard of hearing because we simply do not like the term 'hearing impaired'.
I understand that the majority continues to use the term because it sounds politically correct but in fact it's the other way around.
As for what I've read re merry xmas - I'd just say merry xmas and happy holidays - no need to be exclusive. If you know someone personally who doesn't celebrate Christmas, you can just say "happy holidays" or "enjoy your summer break". If you didn't know, they'd usually say thanks and carry on. No need to sweat over this "issue" imo.