To create a true 5.1 mix, you need all the individual elements. You need the music separate, the dialog, the sound effects, the scene audio...everything. So that you can map where the sound goes in the 5.1 mix. If you only have a stereo mix from a BluRay...or mono tracks...you won't get a true 5.1 mix. Sure, you can (I think) convert that to 5.1...but it's not going to have the low bass, the stereo effect of music....of sound happening on one part of the screen travelling to the other.
If 5.1 is desired, then you need access to all the separate elements...so really the main edit with all the elements on different tracks...from the main edit.