Benedict XVI not the Pope just an opinion?

No, the fact that a heretic cannot be a Pope is a dogmatic fact, a fact inextricably bound up with the dogma that there is only one Faith in the Church and that heretics are not members.  The fact that Benedict XVI is a heretic is also certain since, as we’ve proven, Benedict XVI doesn’t believe that Christ even has to be seen as the Messiah.  He also holds that Protestants and schismatics don’t need to be converted.  These are not opinions; these are facts.  Thus, it’s a fact that Benedict XVI is not the Pope, a conclusion that becomes binding upon all who come across the facts that 1) heretics cannot be Popes; and 2) Benedict XVI believes the aforementioned things (among many others), while possessing a tremendous familiarity with the teachings he rejects. If the position that Benedict XVI is not the Pope is just an opinion, as you say, then it’s also just an opinion that those who are pro-abortion are not Catholic.  But obviously it’s not merely an opinion that those who obstinately support abortion, such as John Kerry, are not Catholic; and obviously one would not be at liberty to hold John Kerry as a Catholic, even though this matter hasn’t been officially declared by the Holy See.  Likewise, it’s not just an opinion that one must believe in Christ and that non-Catholics need to be converted to be a Catholic.  Therefore, just as one is not at liberty to hold a clearly obstinate dogma denier such as John Kerry as a Catholic, one is not at liberty to hold that Benedict XVI (who denies even more dogmas) is a Catholic (and therefore the Pope).  There is no difference.

Pope Leo XIII, Satis Cognitum (# 9), June 29, 1896: “The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium.”