1. In the U.S., "varsity" is not synonymous with "university" and in high school and universities/colleges junior varsity usually refers to the younger or less talented team at an institution. Also, while some would argue the point about being "paid", it is not the case in universities and certainly not in high schools.
2. In the U.S. we would use the term, "horseback riding".
3. "go sailing among the islands" and between the islands of .............."
Your other interpretations/definitions are OK.
Student or Learner