1. There really is no "exact thinning ratio" for this paint. Many factors come in to play when deciding on how thin or thick to mix it, including the type of airbrush you use, nozzle size, psi, and ambient air temperature.
When I mix I usually go for a 1/1 ratio, sometimes I add a little less thinner, sometimes more.
You will have to fine tune your own mixing ratio, one that will work best with your gun. If your gun can handle shooting the paint thicker (my brush cannot, it clogs my tip) by all means go that route, shooting a thicker paint will allow you to apply less coats to achieve the same effect.
I also use a heating lamp to dry my paints in between coats, so i can shoot thin and dry quickly then re-coat, the heat lamp is what allows me to shoot so thin and still get great results on the final product
check out this video (if you haven't already seen it
http://youtu.be/8SRym9nNeMA
2. Currently I am shooting at 60 psi, though this varies for me (and it will for you), I paint in my garage so the temperature fluctuates, and I will have to change my psi accordingly (40-60 psi seems to be the zone I stay in)
3. I would probably avoid a gravity fed gun with this type of paint, As you have noticed, the flake settles to the bottom of your paint cup, I would imagine it would lead to a massive clog up of metal flake in your gravity fed gun . Your flake will always settle, before coats I will give my airbrush a shake to help to re-suspend the flake.
4. and 5. There is no pigment to the paint, think of it as a clear coat with metal flake in it. The color you see depends on a few things, what base color you use (I used black for this car,), what color paint you buy (the flake color), and how many coats you apply.
I used black for the base on the 56 nomad because I wanted a darker/deeper color (even though I wanted to see as little of my base (black) as possible.
In my case I wanted to see nothing but metal flake.
Here is a video showing lighter coats of the metal flake (2 or 3 coats) on different base coats, using the same color metal flake,
http://youtu.be/s3onCclUfC0