1)The "art vrs. theory' seems to be like a wild card on this forum that people always throw into discussions to validate their opinions.....yeah we all know you don't have to be a brain scientist to write a song, paint a picture, write a novel, etc...but sometimes you have to learn things intellectually 1st before you can hear them. I believe a 4yr old child can hear and sing a major scale, penta, etc......but I believe most kids or adults would have a hard time singing a diminished, whole tone, altered dominate, etc, without ist some kind of intellectual training.
2)Inversions...who is to say that the root is going to be doubled? It is not uncommon to double the bass note. Most guitar voicings for inverted 7th & 9th chords do not have the root doubled.
3) A Cmaj13th chord sounds like a A-11/c , it's much to dark sounding for a major chord. Major chord extensions should not go above a 9th, becuase the chord takes on the sound of the relative minor, just like minor chord extensions should not go above an 11th, because it starts to take on the brightness of the relative major.
4) Having an understanding of triads within chords is a crucial for understanding the musical genius of such improvisational giants as Coltrane and Parker....for years I studied the Omini book and wondered how/why Parker broke every theoretical rule ever written and made it sound correct (maj 3rds over minor triads,-7ths over Maj7th chords,etc). I could have just aurally copied what he did, but having the knowledge and understanding of how to achieve this sound makes you a more convincing player. Guitarists Larry Carlton and Scott Henderson are big into triadic improvisation, guitarists who think this way have a tendency to be much more colorful in there improvisations.