How can a father be persuaded to agree to his daughter marrying a man in his forties who is already married, especially since her sisters are not yet married, and how can this man be prompted to formally propose?

If the suitor is suitable and proposes to you, your father must agree. Otherwise, you may refer the matter to a judge or similar authority to have him marry you or order your father to do so. As for the man, he must either propose to you with a lawful engagement and marry you, or else he is seeking what is forbidden, and you must stay away from him and repent for any interaction you had with him.