I understand Ladislaus that a source may not be readily at hand but if a doubt is genuinely established - which is what has been ably demonstrated in this discussion - then that means, for me at least, that I do not believe I would be receiving the Sacraments from a priest 'ordained' by 'Bishop' Pfeiffer even if I am dying. In short, I would see it as a kind of sacrilege.
By definition, doubt means that you MAY (OR MAY NOT) be receiving the Sacraments. If you are convinced that they are positively invalid, then you personally would not consider them doubtful. I hold them to be doubtful rather than certainly invalid ... based on the possibility that there was a legitimate conditional consecration afterward.
If you consider them to be certainly invalid, then there would obviously be no point in receiving them even in danger of death.
Let's say that I currently feel that there's a 50-50 chance that +?Pfeiffer priests are valid. If I'm on the point of death and have no other option, it's OK to roll the dice on this and hope that they are in fact valid. Under ordinary circuмstances, however, that kind of dice-rolling is forbidden. Let's say I was in a state or mortal sin, was dying, grabbed a +?Pfeiffer priest, and made a confession. Now let's say I recover miraculously. Since I considered it doubtful, I would have to go to Confession again, explaining the circuмstances, before I could go to Holy Communion again. Now, if I'm in danger of death, and make this Confession, then I could receive Communion from the same Pfeiffer priest because, if the Communion is valid, then so was the Confession, and vice versa.