Well that is not what the OP said. He said, "If this setup works, I was thinking I could pull out my old g1100 router and connect it to a coax port on the second floor and have that router bridged to my main router so it basically acts as a wifi extender and ethernet hub." I think he is expecting that the G1100 act as a WiFi extender and Ethernet hub. You are correct that the G1100 should not be giving out IP addresses unless the OP is limiting the IP address range in both his routers. Also, the OP wiring diagram shows a MoCA adapter that is separate from the G1100 on the second floor.
Yes and the G1100 will act as a wifi extender and ethernet hub. The DHCP range in the Nighthawk isn’t relevant. If the OP leaves the DHCP server on in the G1100 then it will cause a double NAT. Which is why I told him/her to disable it in the G1100. That way everything will be on the same subnet. The MoCA adapter on the other floor is required for this setup to work. Otherwise connecting the G1100 to the coax on the second floor won’t do anything.