The argument you are making was originally published by the FSSP, so it is not surprising, given your proclaimed adhesion to the PCED position, that you raise it here.
Shawn:
Keeping in mind that the debate is about sedevacantism vs R&R, let's assume for the sake of the argument that you and the FSSPX are correct in your interpretation of these canonical controversies and your understanding of epikeia, state of necessity, etc.
How does this refute the sedevacantist position?
Sedevacantism, as far as I can tell, simply offers a theory as to why they believe a state of necessity exists, or why epikeia can be invoked. Thus the argument itself is neutral with regards to the controversy between sedevacantism and R&R, favouring neither one position nor the other. I suspect this may be why Mgr Williamson, quoting Fr. Roux, states in his last letter that to him sedevacantism is an opinion.