Would a person be sinful if he were to adopt the fatwa of a less knowledgeable sheikh, preferring it for personal reasons such as intellectual acceptance or ease, despite having heard a fatwa from a more knowledgeable sheikh?

A follower (muqallid) is not obligated to ask the most knowledgeable person. Rather, it suffices for him to ask a scholar qualified to issue fatwas. It is permissible to ask more than one scholar about the same issue, especially if there is a justification for it, such as the issue being unclear or doubt about the ruling, as long as it is not for the purpose of seeking out dispensations (rukhsas).