Is a person sinful if they follow a jurisprudential opinion that contradicts what they incline towards and believe to be correct, even though the action is not actually forbidden, and does the ruling differ if the issue is one of disbelief (kufr)?

A person is either a scholar who prefers what appears to him to be supported by evidence, or an ordinary person whose madhhab (school of thought) is the madhhab of his mufti. He must follow whoever he trusts among the people of knowledge. If someone he trusts gives him a fatwa permitting an action, it becomes permissible for him. If he gives him a fatwa prohibiting it, it becomes forbidden for him. He should follow the most knowledgeable and most pious if the opinions of the muftis differ for him. If they are equal, he may choose without intending to seek out dispensations.