Enjoyed your post--especially the bits about independent melodic lines and co-dependences.
I'm endlessly fascinated with independent lines creating a progression of harmonies. There's a life and fluidity with this type of approach, whereas a strict block-chord approach to supporting a melody can be confining. Depends on the form and type of music, of course.
Just this morning, I was reading about orchestration, and specifically about considerations for the foreground, middle ground, and background in a piece. As you mentioned, it's necessary to know the ranges and limitations of the instruments you are writing for. It's impossible to try and study orchestration without considering these overlapping co-dependences, as you put it.
RE: Divide and Conquer – Music Theories