I’ll attempt to answer a few of your questions as best as I can.
Depending on whether you’re editing at Stereo movie or a 5.1 surround movie (and also the method with which you ripped that particular audio) You’ll either have two tracks, left and right, or 6 tracks, left, right, center, surround left, surround right, and Low Front End (LFE).
In stereo, it’s almost impossible to isolate a particular part of the audio landscape. In 5.1 it’s very difficult, and not always possible, but you have a bit more leeway.
There are some videos that have completely isolated center channels that only contain dialogue and maybe a few sound effects here and there, but will have no music. The Clone Wars series is a good example. For instance, you’d be able to completely re-score those episodes by just replacing the music tracks and maybe adding a little bit of Foley work to fill in the surround channels.
Most movies, however, will have muddied center channels, that contain dialogue, music and special effects. Most of the time the music is quieter on this channel than the others, so removing the other channels will get most of the music out. I’ve had some success by doing this and playing with the levels of the videos so that the old music track is barely noticeable.
Star Wars is the most vexing of films because it’s center channel is all over the place, hence why very few of them have drastic audio edits.
I have seen mention of programs that can separate these sounds further, but they are imperfect by nature, and I don’t have any experience with them. Perhaps someone else can give you some more info on that front?
I hope this helps, and good luck on your project!