Is speaking Mandarin essential to gain business customers in Singapore?

It depends on your clientele group, not really essential but good to know as a side note.

It depends on the age group I feel. If you are looking for Gen-Z customers, I doubt it is required.

Yes, I felt like I really missed out because I didnt take mandarin seriously in school. Exposure to chinese markets is unavoidable in your career.

This depends on your business and client types ...