In answer to your question - "If I connect a keypad, M1XOVR, and M1XIN to the ethernet cable, do I need a suppressor on each of those, or just on the ethernet cable."
All 8 leads of the cat6 cable pass through the suppressor at the point where the cable enters the building. Then the cat6 goes from the output side of the suppressor to your box containing the elk equipment. In my case it goes directly to a DBH which then connects a keypad, wireless TX, XIN and OVR.
Instead of adding a second power supply in the garage I added a supplemental power supply in the house and ran a pair of awg18 wires to supply power to the garage which required a second surge protector. My reasoning for this was that this would provide additional power for devices in the house as well as powering the garage.
The supplemental supply is three or four amps which gave me plenty of power for both the garage and house devices. It also has adjustable output voltage which allowed me to turn it up to ~13.5 vdc to account for voltage drop over the long run to the garage. The Elk system can run on 12 to 15 vdc.