Yeah, there has to be some sort of penalty for allowing your turret to get hacked. There wouldn't be much of a point in hacking it in the first place if the owner could just remove it by placing another.
Very true.
Maybe upgrade ally turret just means you can upgrade your own and its considered ally because its friendly?
Doubt it. Allied turret implies an ally's ownership, not a friendly turret which belongs to you (which would be redundant). And I doubt SD would make an objective encouraging a selfish action.
I believe Engineers can repair allied turrets.
At least one of the latest video interviews showed a scene which led me to this conclusion.
Link please? I don't believe I've seen such a scene.