Is it the case that whoever gives a ruling on a controversial issue, in a particular opinion, is considered to not believe in the other opinions, regardless of whether he is aware of the disagreement or not?

The mujtahid (independent jurist) issues rulings based on what he deems most probable according to the evidence. His choice of one opinion does not imply disparagement of other opinions, because those opinions may also be correct, and his followers are even more obliged to uphold this. If we are asked about our madhhab (school of thought) and the madhhab of those who differ with us on subsidiary issues, we say: "Our madhhab is correct with the possibility of error, and the madhhab of those who differ with us is erroneous with the possibility of correctness." As for the muqallid (follower of a madhhab), his madhhab is the madhhab of his mufti (issuer of religious edicts). He is not to condemn someone who follows an opinion different from the one he has adopted. Rather, he believes in the permissibility of following any of the recognized imams in matters of ijtihad (independent reasoning).