Everything about the shoulder is developed in the canvass. Shoulder structure and shape is more about the canvass construction first and everything else second. The cloth follows the line and shape of the canvass.
Shoulder is shaped by the cut in the canvass and how the shape is enforced by padding stitches and iron work to the canvass. Structure and firmness is determined by the weight of the haircloth or whatever other material you use in the shoulder and chest. The haircloth will hold the shoulder line up for extended shoulders regardless of using shoulder pads or not using pads.
Will try to take some pictures when making up canvass this week. Maybe a visual would clarify this