“Who is next in line?” Would be correct to use in all situations except when asking who is first on line.

This is not true. “Next in line for the throne” for instance refers to the first person in line for the throne, unless you for some reason count the person currently on the throne as also in line. When a cashier tells “Next!” they expect to serve the first person in line, not the person after them. You’d think someone was crazy if they said “I’m next” when there’s people before them.

The only scenario I can think of off the top of my head where “next” is not the first person in line is when you add qualifiers to slice the line and refer to the first person after that slice, eg “after Alice, Bob is next in line.”