Yes he was, but if you like jazz, his music is amazing and he did many collaborations. Mark Isham is another jazz musician (and film composer) I like and he is also a Scientologist.
Joke's on me. I never tried Scientology and consequently never made it as a film composer.
The questions are really can you get out of Scientology and stay in the industry, and is getting into the industry worth the strife of getting into and out of Scientology and of association with Scientology in general.
[1] Most affinity groups aren't abusive or dehumanizing, or at least aren't to anywhere near the same degree.
Yes, you can, at least in the last decade. Lots of celebrities who were once vocal Scientologists have now left the church, though they might not make a big fuss about it. Blogs like Tony Ortega's have written on how Scientology's membership is imploding, though it still has some committed donors that it squeezes for money to keep up appearances.