How To Make Repentance Of This Sin
(a) Humans can not confirm or deny this as we have no way of knowing. God take trials, punishes and rewards too. Only God knows truth. (b) Sincere repentance and not to repeat such acts again. (c) It has to be decided by the person according to situation (d) Zina is the main sin. If it has been committed then they should worry about every act that led to it.