My question is that, if someone studies Qur'an and Sunnah without any prejudice, bias or any such ailment and after studying if he reaches to a wrong conclusion and follows it for his whole life (because he thinks it is right and according to Qur'an and Sunnah), then would he be punished for following something wrong, which he thought was according to Qur'an and Sunnah? For example, we have Muslim brothers and sisters who prove from Qur'an and Ahadith that asking help from religious scholars and Prophet (sws) is right whereas they are doing one form of shirk (if we consider their action wrong). Now although their intentions are not wrong but they are doing a grave sin because they think it is in accord with Qur'an and Sunnah and their intentions are right.
Now if we say that because they did not have bias or prejudice and their intentions were right then what about scholars or llamas, because of whose preaching, thousands of people might have been doing a wrong act in their whole lives. For example, Mirza Ghulam Ahmad Qadiyani, now will he be forgiven if he would have a bias less, unprejudiced approach towards religion and still he might have reached the conclusions which he reached? Even because of him millions of people went astray. I mean if a person reached a certain conclusion (which was actually wrong) with a right approach, but because of him a lot of people went astray, would he be forgiven, punished or what?
In principle the proposition you have offered is correct. Everyone is required to follow what he believes to be true after sincere thorough efforts to find the truth regardless of the conclusion. However, practically it is not possible for one who exerts his full efforts after acquiring requisite competence in the religious sciences to reach a conclusion that is totally against the basic teachings of Islam. It is possible that someone reaches a wrong conclusion on the issues which are subject to difference of opinion like for example interpretation of a given text. But there are things which are agreed upon among the ummah have remained so from the life of the Prophet (sws) to this day. We cannot even then decide the fate of such a person who even negates the concept of tawhid for God knows the secrets of hearts and may find the person truthful but we believe that no sincere seeker of the truth can deny that the Prophet (sws) taught this as basic Islamic belief.