Screw it, I'll spoil it.
Later it is stated that Kirito gained the ability duel wield because of his speed stat reaching a certain value before anyone else in the game. Supposedly there are other people with unique skills in the game so Kirito isn't the only player who has one. It is plot convenience to be sure but there's your explanation.
You want to see some real deus ex machina BS? Wait for a few other fights.
On the romantic stuff, I think the conversations Kirito and Asuna have are adorable. I get a lot of amusement out of their interactions. I do see some problems in how they set up some of these scenarios as the progression from point A to point B could be a lot smoother but none the less I enjoy their banter. They act like two teenagers who try to one-up each other and are also too embarrassed to admit their feelings. If that's what the writer was going for then I think he overall succeeded.
And if you don't like the relationship stuff those fights sequences are pretty exciting IMO.
As for "trying to improve" the story presented in the light novels. I imagine they were weary of a backlash for changing things even if it may have been for the better. That's reasonable right? They wanted to keep the intended audience satisfied by not deviating from the source material because someone did buy those novels and liked it as is.