Are the aforementioned sayings—such as obtaining Al-Hoorein (maidens of Paradise) by removing harm, that whistling summons the devil, and that not wearing the Islamic head covering brings the devil to sit on one's head—correct?

The preachers mentioned that removing harm from the road leads to obtaining al-ḥūr al-ʿīn (houris). This ḥadīth (prophetic narration) is not authentic. However, removing harm from the road has other established virtues: it is a branch of faith, necessitates Allah's gratitude, forgiveness of sins, entry into Paradise, and is a beneficial deed. As for whistling, there is no basis in the Quran or Sunnah (definition) that it summons the devil. Its ruling is disputed among scholars (prohibition, permissibility, dislike). Regarding their statement to the nephew that if he does not wear the Islamic head covering, Satan will sit on his head, this is a false statement with no basis in Islamic law. Head covering for men is subject to the custom of people, and if not covering the head is not considered a breach of murūʾah (chivalry/propriety), then there is no harm in it. Head covering is not obligatory or mustaḥabb (recommended) for men in prayer or otherwise. Preachers are advised to adhere to authentic aḥādīth and avoid weak and fabricated ones.