No, you don't need to be engaged to apply for fiance visa however, need to show your intent to get married within 9 months after visa is granted.
No, you don't have to be married to your partner but you must be in genuine relationship. You may be eligible for partner visa when you are engaged or in de facto relationship.
No, English test is not required, but you may be asked by Immigration department how do you communicate with your partner.
No, 820 partner visa is a temporary visa leading you to 801 visa which is permanent.