The Evil Queen's beauty is external, and constantly called into focus by Ravenna herself (internal), while Snow White's beauty is of the "inner beauty" variety, and brought up by others (external).
The fact that the voice of the Mirror is male. Ravenna hates all men save her brother, but in the end, she still internally seeks approval from them by her imaginations of the Mirror. It's implied that it's all in her head rather than an actual magic mirror, which makes it all the more clear that she's damaged goods and more than a little insane.
Kristen Steward seems like a somewhat odd chioce to play the "Fairest of them All", at least against Charlize Theron. (she's nothing to scoff at, it's just the comparision.) But a) is it Snow White's inner beauty that's more important, which Ravenna doesn't see or care about. b) It's the mirror who tells her about Snow White being more beautiful than her. The mirror - unlike her other magicks maybe is not perceived by others, even when she's talking to it with her brother next to her (Finn is not mirrored in the Mirror Man... thing). So, Snow White being declared "The Fairest" is only in Ravenna's head. Why? Because she knows Snow White's the rightful heir and is now grown and old enough to challenge her. Finn probably told her about how stubborn she is... so she translates this fear into the only thing she's consciously afraid of: that she's not the most beatiftul woman (and thus, the one most likelyto come into power with her schemes) anymore. Her perception of Snow White as Fairest Of Them All has jack dung to do with Snow Whites actual looks, one way or another.
Snow White's Raven Hair, Ivory Skin makes perfect sense in this incarnation. She's been locked in a tower since she was a little girl. It makes sense that her skin would be pale and her hair would lose pigments, if she's getting minimal sunlight.
Of course Snow White could be revived with True Love's Kiss. She wasn't actually killed by the apple's poison. It was just meant to subdue her - Ravenna still needed her heart!
Notice how William speaks in a more clipped I Am Very British type of voice when Ravenna is impersonating him. She just knows that he is the Duke's son, so she expects him to speak like a noble. She doesn't know he's a rebellious prince.
William's behavior makes much more sense overall when it's shown that "he" is actually Ravenna. He describes his childhood relationship with Snow White as him being incredibly devoted to her and following her anywhere, while Snow White only remembers him teasing her. He also is considerably more eloquent and forward with his feelings, when before he had a lot of trouble speaking to her. Ravenna really wouldn't know William and Snow White's history. She also heavily relies on seducing men to gain power, so it makes sense that she'd be familiar with what men say to woo women.
Snow White's Adorkable dancing with Gus. She's been locked in a tower for most of her life; she's probably never been taught how to dance, as a princess normally is.
Ravenna sparing Snow White as a child seems odd. However if one takes into consideration that Snow had fairest blood even from that age, it could be the effect of that - even Ravenna couldn't bear herself to kill the princess until she heard what reward it would get her. Further brilliance when you realise that Ravenna's mother was of fairest blood too. She spared Snow because the girl may have reminded her of her own mother.