How can I make the fog lights actually useful by switching them on independently of the headlights?

You can bypass the fog light relay (relay #13). The only function of this relay is to switch the fog lights through the headlight circuit. The full fog light relay current flows through the fog light switch irrespective of this modification, so you should probably exercise caution when upgrading the fog light bulbs. Try a Google search for step-by-step instructions on performing this modification. It really is very simple.  more