Item 1.
We used to have it in one catagory. But hit % gets realy messed up, I.E. you could pad your fighter hit % by just shooting buildings. To fix that problem we made 2 catagoryies for those who do both, and those who just do air to air.
Item 2.
You have some miss perceptions. Proxy kills are only awarded if no one has put any damage on the plane. If you are the only person who shot a plane and it crashes , you will get the kill.
So it already works like you ask, but with anything greater than 0 %.
What most likely happened in the example you gave was some one else had shot the plane up before you arived, you put the final shots on him and he lost his tail.
The person who has landed the most leathality on a plane will be awareded the kill.
If no lethality has been landed, and a plane crashes, a proxy kill is awarded to the person closest, and withing a certain range.
HiTech