I personally wouldn't worry too much about not getting the intimate good-bye. She probably knew the other people before so was more comfortable with them?
I think A and B are related. If she doesn't know that he's poly, then she might be confused about why her son is bringing his girlfriend and "some friend" over for Christmas. Or else, she might have her suspicions (moms are usually pretty clever) and either be disapproving of this lifestyle or upset that he isn't coming clean or both.
But at the end of the day, "awkwardness" sorta comes with the "in-law territory." At least, it did for me :P So I wouldn't use that as a reason not to go...
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker