Hey Adam, a few things I will throw in.

1. You cannot add hardener to a paint not designed for it, so forget about that.
2. You "might" be able to improve your durability with a 2-component (with hardener) style clear coat, but I would strongly suggest that you test a small piece of metal first (before you paint your car) to see exactly how it will affect the basecoat you have.
3. I think I responded over on FBBO to this as well (?). 1-component enamels are somewhat like the old school lacquer paints. They don't have the chemical resistance like 2-component paints (do watch out for gasoline spills), but you might get decent results.
4. I didn't realize that Rustoleum is now making automotive paint.