Ideally, you would have setup with the back elbow pointing back at the back hip. This puts it in a position, that will already have it pointing down, as you use the shoulders to move the arms/club on the backswing.
You may be lifting with that arm or elbow, trying to help on the backswing. If you are, that helping may be pulling the back elbow away from the front elbow into a "chicken wing" (pointing out and behind you) move... then you feel the need to have to get it back or rotate (as you say) it back down. As far as when the elbow bends... it should happen naturally and it does not bend much. You really should not have to think about it if the shoulders are doing their job. Then on the downswing, the back elbow is still slightly bent even at impact... so if you've straightened it out before then, you're using your back hand ... or the arm is doing it on its own. The back hand pressure should not change until after impact. If you focus on the shoulders controlling the backswing, the arms/club/hands just go along for the ride. Takes practice not letting the hands/arms help, but the results are a much more consistent golf swing.
Move Less ... Get Good! DUPLESSISGOLF