Hmm, more Flintstones questions (yes, we know continuity was light in Hanna-Barbera shorts---deal with it...

). To throw up my answers to them:
- Slate Rock and Gravel was a family-run business, and as with most such businesses, one's relatives are often employed as vice-presidents and whatnot. This explains why there's various people named "Boulder", "Granite" and a short, partially-bald guy named Slate in early Flintstone episodes. As for his boss' name, perhaps either the same explanation ("our" Slate has several twin siblings) or he's just plain vain (or just chalk it up to inconsistency if no better explanation).
- Joe Rockhead might be their version of "John Smith"---a common name?
- In real life, many of the traditions involved with or associated with Christmas stem from various pagan/non-Christian celebrations that were common before Christ's birth (including ones associated with the winter solstice), and got absorbed into what we now consider as Christmas activities---things like exchanging gifts and general revelry. Thus, they might just be celebrating the winter solstice and/or the general concept of God, which (like their society's cars and TVs and stuff) just "happens" to resemble our Christmas celebrations (trees, carols, etc.). Why they call it "Christmas" (vs. some rock-pun name or "winter solstice" or something else), I can't say---for the same reason they have cars and whatnot, I guess.
( If interested, Wikipedia has more on such pre-Christianity winter celebrations (such as Yule, etc.):
http://en.wikipedia.org/wiki/Christmas#Pre-Christian_winter_festivals )
Yeah, a geeky answer, but it works...
Related to this, I wonder why in the original series, Fred helped Santa, but in "A Flintstone Christmas", they act like they'd never met (and Santa's dressed like his modern self, not in a Stone Aged outfit like in the original series)... maybe St. Nick erased their memories after the original series episode to protect his "does he exist?" image or something.
"Why do superheroes wear capes?" Because of the influence of the first superhero of them all, Superman, as well as the other early heroes that followed (Batman, the Spectre, Hourman, etc.).