a-hem
anyway, the scoped dart gun is silent, and you don't even need that most of the time, it's very easy to crouch behind enemies and take them out one by one with your bare hands
No, the energy cells are spent after you do. Max of 4 cells with energy regen maxed it will still take you about 30 secs after you are completely spent, any enemy at that point that it's still alive can see the corpses and set the alarm. Plus guards are hardly ever alone.
Trust me, you can't kill everyone and get the bonuses.