Well, you certainly seem to be a bit paranoid about security. But please rest assured. Your system is relatively safe.
1) There's always a possibility
of infection in any system. However, something like 99% of all malware targets Windows, and the chance of a Linux system getting infected is close to zero. There are only a handful of Linux/Unix viruses floating around, and most of those are ancient, benign, proof-of-concept, or just plain don't spread easily. The *nix security model makes the spread of such things exceptionally difficult. I've never experienced any linux malware personally.
security is impossible, unless you unplug the computer from the net, seal up all the ports with epoxy, unplug it, and remove the cpu for good measure.
The only thing you can do is take whatever precautions are available and hope for the best. As I said above, though, Linux is a much more secure operating system than Windows. I've been running without an AV for several years now, and I've seen nothing. Take reasonable precautions, and don't worry so much about it.
3) ELF is just the linux executable format, equivalent to the Windows .exe format. Again, just take reasonable precautions and you'll be fine.
4) Set up a good firewall, keep your system and programs up-to-date, don't use root unless necessary, don't download and open any unknown files from untrusted sources, and relax.