I found that putting Velcro under the shoulder bridge, and then sewing the other piece to the white elastic kept my shoulder elastics from pulling out.
with respect to the snaps pulling through the elastic, I typically double or trooper the elastic where the snap goes through and always burn the hole for the snap post with a soldering iron.
Original photo before the Velcro with elastics pulling out
Side view with the Velcro in place
no more shoulder elastic pulling out
Triple folded elastic snap