Mate, if the low pitch stop sets the prop at 9 degrees or whatever, it will do so if you retard the throttles whether you are maintaining altitude or in a dive a -30 degrees. The governor will command full fine all the way until the low pitch stop kicks in and the low pitch stop will set the prop at exactly the same angle everytime if it is allowed to kick in.
To be able to set the prop at an angle below the low pitch stop you will need to lift the gate or twist the power levers or whatever as if you were selecting reverse on ground. That being said, if you need to set something like 0 degree prop blade angle you will need to lift/twist/whatever the power lever. This would be the so-called "prop disc" on the Dash 8, you can't set it in flight just by retarding the power levers to idle and I don't think you can in any airplane. Well, you can, but you shouldn't!
According to your graphic, to get the "beta angle" I think you need to lift the power levers as if you were selecting reverse in ground. It cannot be done in flight by just retarding to idle