Dude, you know, graphics... They've improved. Maybe Elites mouths-things have always been sharp-y or arrow-head-like, but because of the graphics we've never noticed this.
And about the armor, they're called "Armor Permutations", like spartans, elites have different armors. Maybe this Round-non-shark Star-Wars-Rebel-like helmet is just another permutation.
Could not agree more with you. People always forget about Halo canon. Maybe, when Elites fought about fighting or not the humans, some decided, "What the hell, as we're not you, we're not wearing this black suits, and we're just putting armor over our bare skin". But, I've seen Elites in Halo 4, at least in renders, that still have that leather black suit.