Im not sure what you mean. Everything thats modeled in the lowpoly model is visible in the right screenshot. The rest is made by the normalmap. You can see the details added with it in the screenshot in the middle.
Its mostly just copy/paste of details like rust spots and scratches or use of brushes (I got a set of scratch brushes for example, or grunge stuff). So I dont really paint them but place them in photoshop. Blending modes and other stuff are doing the rest of the work so its no problem to use a mouse there.
You mean those 3 cylinders coming out of the side of the hydrant? It would have messed up the normals and added polygons without any visible difference in the end result. The highpoly had a smoothed edge there so there is smoothing in the normalmap although its just floating geometry in the lowpoly model.