No, the layers are where the actual work goes. Think of it like s stack of paper. Each piece of paper is a layer.
Now Screen is a blending mode. You can change each individual layers blending mode to screen or any other style. Think of the same stack of paper. Now remove one sheet but replace it with a piece of tracing paper. Still the same stack and amount of pieces of paper (layers) but one sheet, the tracing paper (Screen) would look different and any art on it would look different.
Not usually. I guess it depends if your using a pressure sensitive tablet. I just usually set the pen at 20% opacity and also set the pen opacity to pressure, so the harder you push the darker (or lighter on a screen layer) the color is.
Now that is just my way of doing it. You could do it the way your saying as well. Color, then change opacity to what you want. It's a perfectly acceptable way of doing it. That's the thing with Photoshop, there are so many different ways to do the same thing, not everyone does it the same as another.