Re: Experience Points
For healing, consider.
If a player is being healed and doesn't need healing, the experience taken will be negligible, as the player will have only taken say 5% damage.
If they really did need healing, then it was useful.
Maybe it should be faster to kill it and let it respawn, but that's only true for average monsters. The sort people will betray each other over is the really-good-drop, rare kind.
Suggested levels sound like a good idea. I wouldn't use the average, though, because players' effectiveness tends to increase superlinearly with level. More something like ((L1^1.5+L2^1.5+L3^1.5)/3)^(1/1.5) or whatever exponent we determined most closely reflected reality.
Uwillnome: Importance is entirely subjective, though. The earlier player had to keep the creature from regenerating just as well as the new guy did, only he did more than that. If it regenerates, the only person who is missing out is the one who did the damage and then left/died. So, that does give incentive for a player to let a monster regenerate before killing it. But the other way around gives incentive for a player to sit and do nothing. Neither is unexploitable.