That all sounds pretty sensible to me - cupping the left wrist and bending the left arm will obviously lead to over swinging so I think you are on the right path. With my problem (which i will always battle) I was turning my shoulders too much without enough resistance in my lower body. Also bringing the club too far inside the line.

Really concentrating on my posture (which as a begginer i would advise you to concentrate 100% on - name me a top pro with bad posture?? i can name plenty with "bad" grips), and keeping my back leg bent and resisting and not allowing my front leg to get too loose, have really helped with the overswing.

nick