Yes.
Yes, you can because brown primer isn't that dark of a color.
White primer is for use as a primer for lighter colors and the grey is best used as and undercoat for darker colors(blacks, dark blues, dark green etc..). A darker top color requires more coats with lighter color primer.
There are variables such as old used house, or yours just repainting, have there been smokers in the house etc... also, how dark is the previous paint and how dark are you painting with? Primer seals in odors and allows you to paint over dark color paint. It is always advisable.
Typically, you would want to lay down the lightest colors on the canvas and then paint over the light colors with a darker color. It looks better because if you did it the opposite direction, the darker colors would still show through the lighter colors to a degree.
No, you buy the primer separate and paint with it first, let it dry then get the paint and paint over the primer. There are several paint applications where the paint can be used as a primer, and this is called a self-priming paint. In addition, many paints are marketed as a paint and primer in one, however, that refers to its ability to adhere well to a surface and to cover the previous coat. In more difficult priming situations, like staining or painting with a very dark color over a light color, it's recommended to use a dedicated primer followed by a top coat.
As long as the original paint is intact, you do not even need a primer. The advantage about using a primer is that it might help to hide the old color easier, but you will have to buy the primer AND the paint that you like. Is the existing paint latex? Is the new paint latex? Is there any bare drywall? Unless there is something that we don't know, I would just buy enough paint to paint the wall 2 coats. If you want to prime it, you can have the primer tinted a little to get you closer to the color that you have chosen, but it will be more trouble and work than is necessary.
yes
Behr paint covers very well, but for only ten dollars more a gallon, you can get Behr's paint and primer in one. If you are covering a dark color, white, painting fresh drywall, wood, or any situation that requires primer, you can save a lot of time and money by using Behr's paint and primer in one. Behr paint can be mixed in a wide variety of colors, so you can get the exact color you want, with Behr's leading paint technology that gets a clear, rich color every time.
Since it is an oceanic blue color, I'd suggest that you use a primer that is tinted to at least 3/4 of the top coat. This should give enough coverage over the old color to allow you to recoat with only one additional coat. If the primer is not available in that dark of a color, then a gray primer will suffice.
Dark Green
Usually a second or sometimes third coat will cover the white show-through. If you later on wish to change the dark color to a light color, you will definitely need to primer the wall probably with two coats before bringing the color back up to a lighter color.
Yes and using shadowing will add to the threatening feel of the work.