My old drum, but I'm going to beat it :-)
Kills should be awarded 1st) to the person that causes catastrophic damage (loss of tail, loss of whole wing, blows up, etc) and then 2nd) to the person that caused the most damage (enemy plane crashes, 100% damage is scored). Going along with that, once catastrophic damage has been scored, no more damage in caused by anyone shooting at the dying plane.
Beyond that, some random thoughts are...
Next, assists should be awarded and track in a way so that the percentage of damage done translate into a percentage of the kill. Once the total reaches 100%, then that would be tallied as a kill.
A kill should be awarded on any plane that lands and would theoretically unflyable due to battle damage. For instance, if I get shot up and the ditch (loosing my wings, bending my prop), then the person(s) that shot me up should get the kill. The ditch should count as a kill for the enemy pilot, but not a death on the defending pilot. The point is, we are trying to destroy the plane....with or with the pilot, the dead plane is what gets us the kill.
Have the landed kills message show fractional kills.