While I can edit the existing points, there is no way I can move the one I want to move down to ppp (or 0). It continues to snap at pp. Is there a reason for this limitation?
This is just a limitation of the messa di voce dynamic: the control points for the envelope currently only let you control the peak amplitude and then it returns to the original level. If you want more control I suggest that you use the pencil tool to draw in what you need.