I have poor reading comprehension, what does
astroracer say is the hardener he uses?
Any enamel hardener will work fine. You can get some at an auto paint supply or any farm supply that carries paint. Just ask for an enamel hardener.
What's the in-between time for successive coats?
This depends entirely on the hardener used and the amount added to the paint.
Do you need to mix a new batch of paint for each coat, or can you mix enough for say, two coats, and leave it in the cup or a vessel you can seal, like a glass jar, if brush-painting?
The hardener is a chemical reaction. The reaction is based on my above answer. Work time is generally 1 to 2 hours if mixed according to instructions. It will harden the paint if you leave it set so only mix as much as you can use in one application. 
Thanks.