Is it permissible for the preacher to interrupt the Friday sermon to address one of the worshippers, or for a listener to interrupt the preacher with a question? And does this contradict the hadith: "If you say to your companion on Friday, 'Listen,' while the Imam is delivering the sermon, then you have spoken idly (laghawta)"?

There is no contradiction between the two hadiths. The first hadith indicates the permissibility of speaking for both the preacher and the listener if it serves a legitimate Islamic interest. It is not permissible otherwise. Ibn Hajar mentioned that enjoining good and forbidding evil during the sermon does not invalidate it. Ibn Battal said that Umar's statement to Uthman indicates the permissibility of speaking during the sermon for enjoining good and forbidding evil, because it falls under the purview of the sermon itself.