Also, some quick thoughts on last hitting:
Last hitting evolved the gameplay of DotA. This is not yet profitable in Demigod. In the case of Demigod However, an incentive must also be made to actually kill the creeps on the opposing side. If I don't get gold from killing creeps, why bother killing them? The whole reasoning behind last hitting was a duality : First was to get the gold from the creep from the "last hit" as gold was awarded in that fashion. Secondly, last hitting reduced your influence on the creep wave's push, thusly delaying the game's end and allowing you to gain more gold in a more safe gold farming environment in between the first towers of each lane. After all, the towers freaken decimated those creeps, which meant that more creeps from the opposing side would survive for you to last hit. What I'm getting at by saying this is that last hitting wasn't simply a means to an end, but also served as an evolution of gameplay, where delaying the actual win would ensure your ability to win the game through a higher level and more items gained by last hitting creeps for gold and denying your opponent of experience. On top of that, the towers would make it an allied competition over creep kills, where if the computer wins, you lose.
You've pretty much put everything that's bad about Last Hitting in that.
No, you don't need an "incentive" to kill creeps in Demigod - you already have three: Money, XP and having more support than the enemy.
No you don't want to create competition between the each of the players and their OWN TEAM AI. That completely defeats the point. It's your own army. Why the hell would you want a mechanic that makes you fight it.
As for your Ambushing gripes - there already is a significant amount of skill involved in Demigod combat, through items (do you have the right ones?), consumables (can I drink a potion now or will that get me killed?) as well as skills that slow or stun (use them now to get more damage on him, or save it for later to interrupt his teleport/run away attempt?).