I read enough of the spoiler to know I should stop reading any more. :P
I just figured the DW unlock was because he leveled so far using a single blade with no off hand item. Since no other front line player would gimp themselves that way, he alone unlocked it. Dunno if that's actually the case or not, but it didn't seem to untoward with me.
Rather, it didn't seem odd in the context of Japanese MMOs I have played. They don't seem to adopt the Western notion that all classes should cap out at the same balanced level. It's always seemed to be the case that classes that take a lot more skill/effort, or who cost more money, generally perform better. So Kirito unlocking Dual Wielding just struck me as something like that. Of course, I have no basis, so it could be something stupid.
As for Asuna, I've been liking her so far. But the last episode I saw was them deciding to get married, and she was starting to turn into a wet paper bag. Before she had been pretty BAMF though.