"I wonder if a non-major can take lessons with the best teachers, participate in composition seminars for majors, take harmony and counterpoint, etc. etc. "
No. one needs to be a music major or at least a minor to take those advanced classes. And that requires submission of the requiured Thorton application and dmissions material.
One cant take advanced engineering classes without being accepted into Vertibi engineering school.
Why would it be any different for the Thorton Music College.
It is a professional Music school, not just a music program with some basic music classes offered at a LAC.
The OP wanted a college that accepted transfer students, and offered great CS and Music programs in a warm climate,