Is it possible that if Bp. Fellay signs an agreement, then the other three Bishops and the rest of the SSPX say "NO!" and then perhaps Bp. Williamson could consecrate a bishop to replace Bp. Fellay?
Its very possible. But what their strategy will be, no one knows. I doubt they'll do what many of the careless sede Bishops have done and give the apostolic succession to just anyone though.
The SSPX bishops would need to consecrate a devout, pious and knowledgeable SSPX priest.
Apostolic succession is handed down through the bishop so a sede bishop isn't being careless. He's passing on the traditions of the Roman Catholic Church as handed down from the apostles.
The question isn't whether some bishop is a sede or not, the question is whether or not that bishop teaches and preaches traditional Roman Catholicism and instill a love of God. And, of course, whether or not that bishop is validly consecrated. We are living in a time of spiritual disruption and, thanks to Vatican II and Pope Paul VI's new mass, we're living in a time of grave spiritual danger for all catholics. I personally believe that no individual Catholic could ever be considered in error for embracing sedevacantism because working out our salvation in fear and trembling is what each of us is called to do, God willing.
Of course, none of this would be an issue if Vatican II, the great deception, was never enacted.
Usually, and if at all possible, two or three bishops are present to consecrate a new bishop together. For the present 4 SSPX bishops, +Lefebvre and +de Castro Mayer both laid on hands.
I would suppose that at least 2 would be used for any new SSPX bishop, but I would not be surprised if the 3rd is not present, as that would be an opportunity for dark forces to commit a dirty deed. As it is now, all 4 are never present together, by prudent foresight. If they are, they had better have a significant bankroll for super tight security measures.
One of the more difficult and demanding jobs of any bishop is to have about him the perpetual eye of observation to keep tabs on possible candidates for successor(s).
As for Vatican II, if the Catholics of the day had practiced the virtue of fortitude, they would have vehemently protested against the innovations, and the Modernists would have backed off. But that was not to be, somehow. The enemy had made incursions into high offices and the devil winds of change were blowing. It was truly the unclean spirit of Vatican II that made inroads to usurp Sacred Tradition -- which proved to be a much more fragile thing that it seemed to be at the time!