It is difficult to give advice on particular LOR writers since it is a highly fact-specific evaluation. It depends on how well the person knows you, how much they have worked with you, how well they write, how dependable they are, who your alternatives are etc.
1) Forms- It is probably easiest to steer the author to use the provided from. If for whatever reason they are unwilling to use the form, a free-form letter is OK as long as it covers all of the topics of the form. As always, check with the b-school if you have concerns.
2) Who- The title of the author is less important than content (unless the school specifies a specific individual). Based on my limited knowledge of the internship supervisor, I cannot say whether that person would make a good author. Do you feel that this person would serve as your best ambassador?
3) Lead cashier- Maybe. Can she give good examples of your leadership, integrity, problem-solving skills, etc?
4) Clients- Clients are perfectly acceptable recommenders depending on your line of work. Again, it really depends what sort of substantive things this person can say about you.