So, in Hungarian you never use the singular for these types of questions? E.g. "Ki dolgoz a bankban?"
If you want the singular form of this question, you can say "Ki dolgozik a bankban?". This is called " ikes ragozás"/conjugation with "ik".
I think this questions is a very artificial one, so it is rarely appear in the everyday situations. But if you know that only one people is working on the bank and you want to know his/her name, you can use the singular form. If you know that multiple people are working in the bank, you use the plural form ( e.g. you are a police inspector and from the list of suspects you want to have the list of people who are working in the bank. )
No; see the thread above. "Who" usually takes a singular verb in English, unless the verb is only "to be".
Thanks. Now I think I did see it in a lesson somewhere, but couldn't remember it.