Question 1: It's fine to have the inverter ground be the swithc, but better practive to have the +ve switched. If it's already soldered, just desolder it. you can get a de-soldering pump for $2 at radioshack. Not a hard job to do.
Question 2: Nope. Abosultely not. You want a fuse as close to the abttery as possible. Supposed your +ve line breaks and shorts on the car's frame? Fire! Dead battery too. :-)
Anyways, good luck. Sorry to delived bad news :-/