Theologians can have an erroneous opinion favoring BOD and BOB and still be made Saints and Doctors of the Church as I will hopefully show. The concept of BOB and BOD does not claim to deny of the necessity of the Sacrament of Baptism. This is primarily why Popes tolerated and even favored this speculation. This is why the idea was not an obstacle to certain theologians being made Saints and Doctors of the Church. For there are two concepts here. First, "the necessity of the Sacrament" (which is de fide) and second, "how the Sacrament can be received". It is this second concept (invisible reception) which pertains to BOB and BOD and which certain theologians and Doctors taught, and certain Popes tolerated or possibly alluded to in a fallible capacity. Some have also distinguished between "necessity of means" and an "absolute necessity of means" in reference to the visible reception of water. I will now give you some supporting examples...
"We assert without hesitation (on the authority of the holy Fathers Augustine and Ambrose) that the 'priest' whom you indicated (in your letter) had died without the water of baptism, because he persevered in the Faith of Holy Mother Church and in the confession of the name of Christ, was freed from original sin and attained the joys of the heavenly fatherland. Read [brother] in the eighth book of Augustine's City of God where among other things it is written: 'Baptism is administered invisibly to one whom not contempt of religion, but death excludes.' Read again the book also of the blessed Ambrose concerning the death of Valentinian where he says the same thing. Therefore, to questions concerning the dead, you should hold the opinions of the learned Fathers, and in your church you should join in prayers and you should have sacrifices offered to God for the 'priest' mentioned." (Pope Innocent II (12th Century): From his letter "Apostolicam Sedem" to the Bishop of Cremona, Denzinger 388 )
A fallible letter.
"That the place of baptism is sometimes supplied by martyrdom is supported by an argument by no means trivial, which the blessed Cyprian adduces from the thief, to whom, though he was not baptized, it was yet said, "To-day shall thou be with me in Paradise." On considering which, again and again, I find that not only martyrdom for the sake of Christ may supply what was wanting of baptism, but also faith and conversion of heart, if recourse may not be had to the celebration of the mystery of baptism for want of time. For neither was that thief crucified for the name of Christ, but as the reward of his own deeds; nor did he suffer because he believed, but he believed while suffering. It was shown, therefore, in the case of that thief, how great is the power even without the visible sacrament of baptism, of what the apostle says, "With the heart man believeth unto righteousness, and with the mouth confession is made unto salvation." But the want is supplied invisibly only when the administration of baptism is prevented, not by contempt for religion, but by the necessity of the moment."(The Seven Books of Augustin, Bishop of Hippo, On Baptism, Against the Donatists, Book IV, Ch 22)
Augustine retracted this opinion later on.
Another example would be Ludwig Ott who on pg. 356 of his book Fundamentals of Catholic Dogma, he states...
Baptism by water is, since the promulgation of the Gospel, necessary for all men without exception, for salvation (De fide)
then he says...
In case of emergency Baptism of water can be replaced by Baptism of desire or Baptism of blood (Sent. fidei prox.)
How can he say this, however, after stating that it is "de fide" that Baptism by water is necessary for all men without exception, for salvation? He just stated that water was absolutely necessary for salvation and that it was de fide! He would be condemning himself here!
This is because, in the warped mind of scholastic theologians they are not denying the necessity of water baptism without exception, for salvation. They are merely saying that the Sacrament can be RECEIVED "invisibly". This "replacement" spoken of by Ott, is merely a replacement of the visible Sacrament (water) with the invisible Sacrament. This invisible reception is not considered to be de fide by Ott, however.
But how do they explain this Canon from the Council of Trent?...
CANON II.-If any one saith, that true and natural water is not of necessity for baptism, and, on that account, wrests, to some sort of metaphor, those words of our Lord Jesus Christ; Unless a man be born again of water and the Holy Ghost; let him be anathema. (Council of Trent, Session 7)
They merely say that true and natural water is of necessity (as opposed to some other liquid), but not necessary as per the VISIBLE reception of such.
So the Popes who tolerated or supported BOD and BOB and proponents of these errors are mistaken in regards to the reception of the Sacrament (water Baptism), and not its necessity (it seems). The point of doctrine in regards to the invisible reception of the Sacrament of Baptism is still up for debate according to the Popes who tolerated it. It is a distinction which has not yet been postively condemned.
Also, it is important to note, that in regards to BOB and BOD for Catechumens, little changes in the practice of the Church in regards to baptism. The Church has always delayed the administration of water baptism for certain individuals. In case of emergency, water baptism was always supposed to be given. This is because according to theologians who held the false theory of BOD, perfect contrition would also be one of the requirements for salvation in those desiring baptism. The Church does not presume to know who is perfectly contrite and who isn't. I am not making an excuse for BOD, but am merely showing how a given Pope may be inclined to tolerate the false opinion or even teach it fallibly when it applies to Catechumens.
The problem occurs, when holders of this false theory try to figure out exactly how a Catechumen becomes part of the ecclesiastical body of the Catholic Church and one of the faithful. For one, it can't be done with contradicting Catholic teaching. One cannot be part of the soul of the Church without being part of the body. One cannot be saved unless he has remained in the unity of the body (Pope Eugene IV-ex Cathedra). One cannot be on the porch and be in the house (at the same time). One cannot be a Catechumen and be one of the faithful. Catechumens beg for the faith that bestows life everlasting (Trent). They do not have it. They only have human faith (belief), which is required to dispose a person to recieve justification, but does not justify. They recieve it in the Sacrament of Faith (water Baptism). You need to be BORN AGAIN to be justified and attain salvation. BOD does not produce this effect. It does not regenerate. Secondly, once you have devised a way (a doctrine of man) to admit Catechumens into the Church, you potentially open up the same possibility for Pagans, Jews, heretics etc., which is exactly what has happened. This is why we can't make excuses for not fighting against BOB and BOD. Unfortunately, theologians (even Doctors) can err in these matters. Particularly scholastic theologians, doctors and Popes who love to speculate on how things may be possible. Doctors are not infallible. Popes are not infallible in their opinions and inaction.