Yes, a higher perception can help, or mousing over them or moving an object that is covering another, etc.
Sneaking works the same in combat as out, though higher levels do not reduce the AP cost to sneak, so it is more useful for initial positioning than as a tactic to use repeatedly.
Yes. When you mouse over a target to attack, an indicator will show a cone where you need to stand to be behind it (if you can backstab).
