nah, overthinking it imho.
no need to go through the face. If its 3/4" MDO, 3/4" screws should work since the thickness of the angles will keep you from drilling through the face (maybe add a washer if you're really worried about it).
Just use a few extra screws than you think you might need, or maybe another set of angles on the other side of each post.
Gold screws or deck screws will work just fine, silicone not necessary.