1. They make two kinds of valves. One is temperature control only and needs a downstream shut off to control the flow. The other has the on/off function built into the valve. Which do you have.
2. If yours does have the on/off function, then why do you have the second control valve, and the main valve should not leak any water unless it needs to be repaired.
3. When a multiport diverter is installed after a valve with its own on/off function all the ports have to be used even if it means connecting two of them to the same shower head device, to prevent using it as a shut off, but it is more to prevent using that control as the shower on/off valve than to prevent back pressure.