Rav Chaim explains that is what Tosafos meant that we consider the minority opinion is as if it is not there. This is because once a majority of the judges rule one may we see the entire beis din ruling in that way.
Based on this we can answer the Rashash’s questions. The Torah set up that there should be lower batei denim, which have the authority of a beis din. When such a beis din has a majority opinion, it becomes the unanimous opinion of the beis din and now the beis din as a whole rules in that decision. Therefore, there is no reason to go to the Beis Din Hagadol when there is a minority opinion in a beis din because it is as if the lower beis din issued a unanimous opinion.
Similarly, there is no reason to search for a reason to acquit a man found guilty of a capital offense after a beis din of 23 has found him guilty even though there were several judges who believed that he was innocent, because now we consider those opinions obsolete.
Finally, we likewise do not have a problem of following a rov when it is possible to discern the truth by other measures, because in this case we are not following the rov, rather it is rubo k’kulo. There is no minority opinion. Therefore we do not have to take the case to the Beis Din Hagadol.