History is strewn with examples of men who have fallen into reactionary positions that are as equally has harmful as that against which they had reacted.
Speaking hypothetically, let's say Benedict says something like "Christ is not devine" or "Mary was not born without original sin". What should we do if such an undeniable heresy were to be uttered? Not trying to start anything, I'm just curious what your answer is. Would you say the reaction from laymen should be the same, or would you consider further action?
A layman can not immediately do anything in that case except make your concerns known to the Holy Father. The church provides a procedure as a remedy in such a case and it does not involve the laity. It is up to the Bishops and sitting Cardinals to bring the Pope to an understanding of his heresy. If the Pope refuses to abjure his heresy and the remedies have run their course, it is up to the hierarchy to formally depose the pope.
Your question should be, what if the hierarchy refuses to bring the pope into this process, or, what if the hierarchy agrees with the pope. As a layman, the only choice you have to make is to not follow the heresy, as you are bound to reject it. There is no precedent for a layperson to personally adjudicate the heresy and depose the pope. But one certainly has an obligation to reject the heresy of the pope and/or any other priest, bishop, or angel that holds that heresy. And then, you must continue to remain Catholic in the same way you are now doing.