How can I make soundtracks with one artist sort as soundtracks in iTunes (and iPhone)?
You've pretty much figured it out. Just pick a consistent description to use for the Album Artist or Sort Artist and your albums will group together, while being able to preserve the artist for a specific track. The de facto standard for this seems to be calling it "Various Artists", but there's no reason you can't do whatever suits you, whether that's "Soundtracks" or a specific title for each individual soundtrack album.