In OpenGL, here's how them matrices are composited when going all the way to clip-space:
PROJECTION * VIEWING * MODELING * vobj = vclip
As you can see from this order, if you put something on the PROJECTION matrix after the projection, or if you end up putting on the MODELVIEW before the viewing transform, you end up having it in the same order.
As to your latter question, here's a good post on this. In-short, the space between the PROJECTION and the VIEWING transforms (called EYE-SPACE) is important and used for various functions in the pipeline:
Also, I haven't traced it through, but I'm not sure your "scale on the projection matrix" trick will work with a perspective transform. Probably safer to tweak your projection transform inputs if you really want to change the frustum.