1. I assume you have a color-cup (not the bottle). I'd fill it with about 2/3 to 3/4 with your thinned paint and that should be plenty for a 1/72 fighter. If you're using a bottle, then you shouldn't need more than about 1/4 inch in the bottom of the bottle. However, if you spray it on really thick, then the paint will go much quicker. I find with airbrushing the trick is to spray light 'mist' coats rather than lay it on thick (especially in 1/72 scale).
2. Depending on the color, 3 or 4 light coats should do the trick. You might be able to do with less. Use your judgement after each coat.
3. I recommend putting on Future instead of the Gloss lacquer. It's much easier to work with and has way less risk of damaging the underlying paint. You'd decal overtop of the future, then put the flat over-top of that. However, I don't think I'd do a flat LACQUER coat over top of the future, because I'm not sure if that would react well. I'd get an acrylic flat and put it overtop (GS, Tamiya, and Microscale all make a flat finish that shouldn't harm the future.)
The best thing you can do is get a scrap piece and try everything out together. Better to find out about a 'problem' on a test piece rather than on the real thing!
M.