2) This is a DC Surge suppressor. It gets installed between the +56 VDC supply and the CMM4 inputs. The CMM4 cannot accept -48 VDC, you'll need a DC-DC converter to get the voltage to at least +48 VDC prior to connecting to the CMM4 itself.
3) DC surge suppression is highly recommended to protect against DC surges. Reminder that you'll need DC-DC converter in order to utilize a -48 VDC supply. We have recommended the SD-200C-48 for this purpose.
What kind of radios are you attaching to the CMM4?
We have also just released the CMM5, which can accept -48 VDC power natively, and has gigabit ethernet ports. You may want to consider this product instead.