Back doors are often your own creation by not setting up strong enough passwords and/or using the Admin user account instead of setting up user accounts that are limited for day to day use. Then only using the admin account to maintain the system. Also setup required password on boot up.
Root kits are OS break-ins. As to the risk while it's not zero its as close as you can get running OS-X Lion or newer without another OS running on your system. Other OS's are much more risky.
Anything running in RAM gets lost as soon as you power down your system (but not in sleep mode).
As to Firmware sure its possible for someone to alter it but they would need direct access to your system to do that (no means to do it remotely).
As to your system being exposed just being connected to your Ethernet network, Yes, your system is broadcasting its name across your local network so it can be seen and someone could also probe your IP network finding your system from the internet. So yes you are exposed!
Now crossing the street or driving a car could be dangerous too! but we don't panic on that risk. Basically, I think your over doing it here.
You just can't live with zero risk, but you can be smart!
Don't download apps from pirate sites, don't open email that you don't know the sender. Install and maintain a good anti-virus app and use it to scan your HD often. Maintain your OS & Apps, Have a good backup setup so you can recover things. Setup a good router and maybe even a double Nat'ed setup (2 routers). Enable Find my Mac and consider setup a hardware password. Use user accounts effectively. Don't leave sensitive data on your laptop and use encryption on it so its not easily readable and lock it so it doesn't walk away on its own (Kensington laptop lock).
Lastly, enjoy life! Don't cower in fear, life is too short.