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.

I'll try to find sources.  They have been cited here before.

No, this is not negative doubt.  It's POSTIIVE doubt, since there are concrete reasons for the doubt, i.e. the botched consecration attempt ... captured on video.  Negative doubt resolves in the practical order to moral certainty with regard to the Sacraments, whereas positive doubt does not and is distinct from certainty.  You make a gratuitous assertion regarding the nature of the doubt, implying that if one is not certain that they are invalid, then that's the same as negative doubt.  It is not.  Similarly, I am not certain that the NOM is invalid under all circuмstances, but I do hold that there's positive doubt due to the concrete, specific, and credible arguments made against it.

I think that if you find yourself with only a "bishop" Pfeiffer priest available at your death, then your salvation is already in serious doubt -- it would suggest that God has begun punishing you and signaling that you weren't a very good guy.

Yes it is negative doubt. You may have positive doubt regarding the ‘botched’ consecration, but you want to extend this to the conditional consecration too. Following this line one could extend this “positive doubt” to any subsequent consecrations by Bishop Webster (or, indeed, antecedent ones).

It easily extends to the conditional.  Father Pfeiffer went on and on about how the first one was valid, which immediately makes his judgment on the issue questionable.  So if all we have to go on is his personal assertion that the second one was valid, he no longer has credibility.  We actually have no proof that the second one even took place as alleged.  Father Pfeiffer has lost credibility as a witness due to his behavior over the years, including his persistence that this Ambrose character was a valid bishop despite all the evidence to the contrary that was put to him.  He's shown that he simply wants to believe everything is valid because he wanted to become a bishop.  That is plenty concrete.  It's not in the realm of "what if [any given unknown] priest at the altar botched the words of consecration."  All it takes to render doubt positive is a rational credible POSITIVE reason (something you can point to) rather than a "what if?".  This is well beyond the realm of what if.

No, I agree with Lad that's a positive doubt- a doubt based on a reason. Doubting a sacrament because "something might have been done wrong but I have no particular reason to believe anything specific was done wrong" is a negative doubt. Here, there are specific reasons to doubt, including the minister's facility with Latin as demonstrated that day or the day before.

Now, a positive doubt is not necessarily a valid doubt just because it is based on some reasons. Contrary reasons or evidence are also considered. For example, omission of words ("mystery of faith") may be a reason for a positive doubt about the NO consecration, but it's not necessarily a valid reason for positive doubt when its role in the form is considered.

I'll try to find sources.  They have been cited here before.

No, this is not negative doubt.  It's POSTIIVE doubt, since there are concrete reasons for the doubt, i.e. the botched consecration attempt ... captured on video.  Negative doubt resolves in the practical order to moral certainty with regard to the Sacraments, whereas positive doubt does not and is distinct from certainty.  You make a gratuitous assertion regarding the nature of the doubt, implying that if one is not certain that they are invalid, then that's the same as negative doubt.  It is not.  Similarly, I am not certain that the NOM is invalid under all circuмstances, but I do hold that there's positive doubt due to the concrete, specific, and credible arguments made against it.