[jack halsey]"it seems to be that the whole "mix" can be controlled via gain or the volume rubberbands to fade in or out the music."
Well, yeah, that is the bare bones of mixing, but if that was all there was to it, then a lot of audio guys would be out of work.
[jack halsey]"I still intend to run it thru dolby speakers etc"
I don't know what you mean by that.
[jack halsey]" dont see it as complicated as mixing tracks of a music composition"
It is the same thing, only your elements of the mix are different. A good audio person will carve out holes in the music with eq for the dialogue to sit in, will ad compression to the elements if needed, will eq and ad reverb to scenes to help match the acoustics of the scene before it and the one after to create a seamless soundscape. It ain't just riding levels. If this project is important, (and aren't they all?) then I would hire an audio person to do your mix. Just the nature of your question tells me that you don't have the experience to do justice to a film.