5Too many questions for one post. I have answered some.
1)"A Merry Christmas and A Happy New Year"
2)"Merry Christmas and A Happy New Year"
3)"Merry Christmas and Happy New Year"
You will hear all three. On the rare occasions I utter such words, I use #2
What is the difference of "Merry Christmas" vs. "A Merry Xmas"? Some people prefer the full form, feeling 'Xmas' is lazy and/or inappropriate for the celebration of the birth of Christ.
Why are the articles capitalized (while "and" - not)?
In titles, headings and greetings the 'little words', prepositions and determiners, are rarely capitalised.
Really, these holidays seem to me like known definitive specific events
Can I say:
"The Merry Christmas and The Happy New Year"? N0