Do I have to add more that one choice for a question to practice it in "Quiz" (Multiple choice) mode?
No. We assume, that all questions in one quiz are related, so the answer for one question, can qualify as the wrong answer for another, yet it is relevant to the subject. So you can add only one (correct) answer for a question.
Also, worth mentioning, "choice suggestion" will suggest a choice as you type.
Please describe the matter of abuse