bendport

If you look on youtube at the best players swings they start the downswing with a lateral bumb of the left hip followed by a rotation of the hips, here is a drill to show you how to do it easily

The best way in my opinion to start the backswing is with the right wrist hinging, this will then pull the arms and in turn the body. Here are two drills for you, one is to keep the hip from sliding and the other is for how to take the club away, combine them both and this will give you a good feeling for the backswing. Make sure your spine is tilted enough to the right in your set up as this can cause the hips to sway laterally to the left in the backswing(Reverse pivot)

Difficult to comment on without the seeing a video of your swing but it looks to me like the problems are occuring before impact as your hands aren't ahead of the club head enough meaning that you have lost some lag in the downswing. It also appears to me that you may be pulling the left arm and shoulder back around the impact area which is preventing the left arm to fold correctly. When you swing, scan the tension in the left arm and shoulder around the impact area to make sure you are not tensing up then check out the drill below which will help you to increase lag and fold the left arm correctly on the follow through.

Read Bob Rotella "Golf is not a game of perfect"

I would try the STEP IN DRILL. What you need to do is Stand away from the ball, swing back then step into the ball with your left foot as you simultaniously hit the ball. The momentum of the step in will force your posture angles to be maintained or you may even tilt over more on impact. This will give you a good feeling of maintaining your posture angles through impact.

Try the throwing drill as it will give you the feeling of the bump and the natural rotation at the same time

Totally agree with you. If you can bump your hips laterally then they will naturally rotate open on the throughswing. A lot of golfers try to rotate their hips open to start the downswing causing them to swing over the top. Here is a nice simple drill to help you get the move you are describing.

A good forward shaft lean on impact is a bi product of getting some other key things in place in the swing. Using an impact bag can give you a stationary feeling of impact but if you don't have these other important aspects happening in your swing you will never get into the correct impact position. It's my belief that getting a good forward shaft lean on impact is created by 3 things.... 1) Maintaining a tilt of the spine to the right at the top of the back swing (Face on View) If your spine tilts to the left at the top of the swing you will lose your body shape and you will have to cast the club from the top to widen the downswing arc to stop you from topping the ball 2) A good lag on the downswing. Once you've got the correct body shape at the top a good lag on the way down will help you to get the forward shaft lean on impact. 3) Maintain your posture angles through impact If you straighten your spine angle (Down the line view) you will lose your lag and flip the club on impact as you will have to extend your right elbow and wrist too early to avoid topping the ball. Any thoughts or comments?