I wouldn't use "retail business" in any of them. In BrE, it's perfectly acceptable to just use "retail". Sentence 1 is correct. I think sentence 5 is OK in AmE but in BrE we don't use "work a job".
The simplest and most natural versions, for me, are "Many college students have a part-time job in retail" or "Many college students work in retail part-time".
Student or Learner