Two things to consider.

1. Scale effect. You always want to paint an object lighter in color than it's actual color in 1:1 scale.

2. Weathering. Washes and filters and all forms of weathering will darken a vehicle. If you start off with very dark paint, it will become an extremely dark mush by the time you are done.

When I use the Lifecolor set I use the darkest shade first, a lighter shade to highlight the centers of panels and any areas between raised detail, and then the lightest shade moved in even tighter to the centers after that.

That depends on what that black primer is. If it's enamel based, you should really let it fully cure before going over it with acrylics. If Citadel is truly an acrylic, you're good to go.

As far as the lifecolor paint (or any acrylic for the most part), I apply the next coat once the first is dry enough to handle and have never had issues.