I then... roll the fabric over the foam... I'm making all of this sound easier than it is.
The difficulty I had turning the first version of the shoulder straps right side out after sewing and then stuffing the foam is the reason I decided to sew with the right side out and then hide the seams with grosgrain. Even with this method, it still took me over two hours just to stuff in the foam for the hip belt. I wish I'd thought of rolling the fabric onto the foam. I think I'll stick with sewing right side out, though, for two reasons. 1) I really like the look of the grosgrain edging, and 2) I think the taper of the hip belt will make the next one a lot easier to stuff.
I'm going to take your advice about sewing the webbing on and sewing through to lock it all together. I think my machine should be able to do it with the foot raised if I turn it by hand.