In all honesty, I would give enamels a good 5 days minimum. It's really worth it in particular if you are going to mask it. Just find something else to do. Often I'll paint a bunch of models at once. Say you have cockpit green or tire black in your air brush. Go though all your unbuilt kits and spray the parts on the sprues. Even though you may well later paint them again or touch up the points of attachment with a brush. it's a good way to be efficient with paint and clean up.
Stripping and repainting is really the seventh circle. And touch up repairs are really hard.
Sure, mixing paint works well. Usually you want to darken a mixture, not lighten it.
I'd start with a little white and add your gray to it. Otherwise you may end up adding a lot of white and get a cup of paint before you are happy with the results.