Is it obligatory to ask for forgiveness from a person who was physically assaulted, knowing that he is among the people of innovation (Ahl al-Bid'ah), and he had previously forgiven, but it is feared that the forgiveness was merely out of courtesy?

Your assault on the man is a forbidden injustice. If he did not do anything that warranted being struck, then the fact that he is an innovator does not permit you to assault him. If the man forgives you willingly, that is sufficient. However, if his forgiveness was out of shyness or courtesy, then it is as if it never happened, for what is taken out of shyness is like what is taken by force. You are not obliged to go back and ask for forgiveness. The innovator should be advised, and warned against innovation.