EXCELLENT idea on waiting until you get the chest and back complete and fitted. That way you will have a better idea on where to place the bells.
I assume you mean the white elastic that connects the chest and back, correct?
If so, doubling this for added strength is certainly can't hurt. As for adding a strip of nylon webbing, I don't think it will be necessary. BUT, if you do, make sure the posts on the snaps will be long enough to go through all the layers and still have enough for them to connect securely to the bases- you might want to do a test piece.
Keep those photos rolling in!