Will the cases you purchased hold stamps on both sides?
Cling Mount Red Rubber - They will only hold stamps on one side. They aren't thick enough for cling mount stamps on both sides.
Clear Stamps - yes, you could put stamps on both sides. The cases will close easily.
Yes, there is an opening along the hinge side. It is 1/2" wide and the length of the case. If you put cling mount stamps in it, they cling to the case, so it shouldn't be a problem. I have tested some of the smaller Spellbinders dies and after shaking the case, they can't get the angle to fall out. I think I will be adding a strip of the magnet sheet to hold the smallest dies in place though.
Will these cases fit the Borderabilities Petite and the Mega Curved Rectangles? Yes. The borderabilities will fit, the long way, but not horizontally.