What's the recommended way to install the wall mounting spacer?
I would try to gently compress them in with pliers (ordinary slip joint or channel locks) or a clamp (e.g., a C-clamp).  Use a scrap of wood as a protector on the outside to prevent damage to the finish. You might have to alternate from one pin to the other until both are easier to push in. Do the pins or the holes have a means to avoid resistance from trapped air?