Fr. Pfeiffer's episcopal consecration was long discussed here on this board when it took place.
The consesus was that it was doubtful, if I remember correctly.
It would be wise to revisit that thread if you are seriously considering going to Fr. Pfeiffer's to receive the sacraments.
About Bp. Williamson, I have never seem him disapproving the doubtful Novus Ordos priests who were admitted to the SSPX without a conditional ordination, so, unfortunately, his opinion on the validity of sacraments must be considered with great caution.
Personally, I doubt he is a bishop, but if he is a bishop he is in big trouble with Our Lord - that's my opinion. But we can and must get to Mass to fulfill our obligation to the Church or we sin. Too many trads do not take this obligation as serious as they should imo. My opinion remains, I will stick with the law of the Church and attend his Mass if that was all I had. I would not recommend listening to his council in confession unless it was soundly Catholic, but I would go to get absolution from him - that's what matters.
We do not need to receive his communion, that obligation is for us to receive communion once a year - or we commit a mortal sin, that's another of the Church's laws. We're supposed to do what we can to abide by her laws whenever we can, and in this case of QVD's scenario, we can.