Is it considered sinful to engage in jurisprudential discussions, such as the prohibition of singing, and to express an opinion that contradicts a revered scholar like Ibn Hazm, even if the religious evidence and the sayings of the Salaf support the opposing viewpoint?

All singing that contains obscenity, iniquity, or incitement to sin is forbidden. Scholars are the inheritors of the prophets, and revering them is revering Allah. The Prophet warned those who do not acknowledge the rights of scholars. Scholars are not infallible, and they are rewarded for their diligent efforts, even if they err. If you attribute an error to a scholar whose error has been proven, and you do not intend to diminish or disparage them, then there is no blame upon you.