Unless I'm not remembering, TG didn't really devote much ink to the Hollywood film scoring scene and its relation to west coast jazz. Considering that many west coasters played on film and TV soundtracks and that some of them were composers themeselves, I thought that was odd. While I haven't read anything confirming this, I have always suspected that the proximity of Hollywoood and that the diversity of film music in the 1950s and 60s must have had some influence on the compositional and arranging aesthetics of west coasters.
I think you are right on both counts.