"if virginity were not found in the maiden": i.e., if there were no witnesses to refute the witnesses of the husband.
(Ibid. 21) "Then they shall take out the maiden to the door of her father's house." This tells me only of her having a father and a door of her father's house. Whence do I derive (the same for) a maiden who lacks either one? From "Then they shall take out the maiden" (to be stoned) — in any event.
(Devarim 22:21) "and the men of her city shall stone her": Now do all the men of her city stone her? (The intent is that she shall be stoned) in the presence of all the men of her city.