I'd call that person a shop assistant. A clerk performs tasks more related to paperwork, e.g. a bank clerk. This applies to British English. American English differs.

We have no specific word for that. The word to use would depend upon context. How do you intend to use the word? To whom are you speaking? Will the person selling the bread be listening to what you say? I'll give one example. Suppose I am in a bakery and I am with a child. I want the child to pick out a confection for herself. I would say to the child "tell the gentleman (or lady) which cake you prefer". "Gentleman" is a good word to use because he hears you, so you want to be polite. There are other choices for different situations: "employee", "assistant", "baker", "helper", or "cashier" (if they work the register).

While they are standing behind the cash register, if they are not the baker or store manager, they may simply be a cashier. Clerk would be a common term in American English. I also like baker's assistant.

This is a good question. I often just say "The man behind the counter." (But this is not concise) If they are taking the money, I might refer to them as the cashier. In America, we sort of dance around this issue with various expressions, haha. Good luck!

Depends. Maybe they helped bake the bread. In this case, they would be a bakery assistant or a baker, if they own the bakery, perhaps, and bake the bread themselves. Shop assistant is correct also.