Would it be considered an act of disbelief (kufr) or merely a sin if places in one's father's house become ritually impure (najis) due to obsessive-compulsive disorder (OCD) and masturbation, and then the Qur’an is placed on a pillow or in a closet that is ritually impure?

Semen is pure (tahir) and not impure (najis), but it is considered repulsive (mustaqdhar), so it is not permissible to intentionally stain the Qur'an with it. A Muslim is pure, even if in a state of ritual impurity (junub), as the Prophet, peace and blessings be upon him, said: "Indeed, a Muslim does not become impure." The obligation of ghusl (ritual bath) is not because the person in a state of janabah is impure, but rather it is an act of worship (hukm ta'abbudi). As long as the questioner reveres the Book of Allah, he is upon goodness, and he should abandon obsessive thoughts (waswasah).