Question: if i dmg a mob but my party member kills him instead, do i get the full percent HP return, a partial return based on dmg done, or no return in HP?
No, you don't receive a return in HP.
On a larger scale, this is something we've been discussing how to improve. We definitely realize the way Life on Kill works right now isn't very co-op friendly. When you're playing with a group of people and sharing your killing blows, the affix just doesn't have as much value as it does in solo games, and that can be a really poor user experience. We also recognize the same applies to various skills, such as the wizard's Archon ability.
We still want players to feel like they can gear/build their characters differently for solo vs. co-op, of course, and we think there will always be some affixes and skills that are better in one more or the other. But, at the moment, the degree of advantage seems like too much of a swing (I imagine a lot of players would agree). We've been discussing possible solutions for how we can bring LoK/Archon/etc into better balance between the two game types, but haven't settled on the best course of action just yet. This issue is absolutely on our radar, though, and we're certainly open to your feedback in the meantime.