Asuna said that roughly 500 players take part in the frontlines, because the rest became comfortable with SAO now after 2 years, it doesn't mean there are 500 players left. I'd say there are around 50% remaining out of the 10.000, which would be around 5000 players remain alive.
There probably are more solo players other than Kirito, but Kirito is on the frontlines, we don't know how many solo frontline players are there but being solo there is reckless so I don't think there are many of those. Kirito might be the only Solo Frontlines player, but it's unconfirmed.
I don't think Kirito is the strongest player but I would say Asuna is close to his level, unless he's just at this age that he is struck by her beauty, and just has definite hots for her. If the are at the relatively same age, she should feel the pressure soon as well.