And will Williamson remain a bishop, return to being a priest, or be defrocked?
In the sacramental sense, yes. Of course.
In the practical sense, I think not. In fact, I won't be too surprised if, should the bulk of SSPX formally reconcile with the Church, Williamson departs from SSPX and starts his own religion with himself at its head. It wouldn't be the first time something like that has happened, and he seems to have the right combination of nuttiness and arrogance. Of course, the alternative of obeying Fellay's order to STHU and ending his life in quiet obscurity (for the good of the order and of the Church) is also possible.
The big picture, here, is a lot bigger than one recalcitrant, illicitly consecrated, antisemitic nutcase.