With regards to bias and opinion, I think that we must try to distinguish between the two. They seem to be different words to me. I would consider bias an influenced viewpoint that deviates from fact or clouds the reader’s judgement of the truth. On the other hand, opinion is a personally held belief, that could be fact or opinion that is not concerned with giving an impartial account of something. In this sense, one’s opinions could affect their account of an event and cause it to be biased, but the two words are indeed different things.
To use an example, let’s think of a religious person that believes in creation. I would say that it is their personally held belief (their opinion) that a higher power created earth. If they tried to write an account (or history) of the beginning of the earth – I would call that account biased. To be sure, their are instances where the difference between bias and opinion are not so clear but this is how I distinguish between the two.