So all the theologians who say to
Deny BoD is a mortal sin, you get around this...how?

Everyone should read that article on Limbo in The Catholic Encyclopedia:

Besides the professed advocates of Augustinianism, the principal theologians who belonged to the first party were Bellarmine, Petavius, and Bossuet, and the chief ground of their opposition to the previously prevalent Scholastic view was that its acceptance seemed to compromise the very principle of the authority of tradition. As students of history, they felt bound to admit that, in excluding unbaptized children from any place or state even of natural happiness and condemning them to the fire of Hell, St. Augustine, the Council of Carthage, and later African Fathers, like Fulgentius (De fide ad Petrum, 27), intended to teach no mere private opinion, but a doctrine of Catholic Faith; nor could they be satisfied with what Scholastics, like St. Bonaventure and Duns Scotus, said in reply to this difficulty, namely that St. Augustine had simply been guilty of exaggeration ("respondit Bonaventura dicens quod Augustinus excessive loquitur de illis poenis, sicut frequenter faciunt sancti" — Scots, In Sent., II, xxxiii, 2).
...
As to the difficulties against this view which possessed such weight in the eyes of the eminent theologians we have mentioned, it is to be observed:

Modern theologians make the same mistake that Bellarmine et al. did ... thinking wrongly that the speculative theology on BoD "intended to teach no mere private opinion, but a doctrine of Catholic faith".

AND ... the Council of Florence definition is easily reconcilable.  Limbo is in fact, strictly speaking, part of hell ... and the "widely different penalties" does not rule out that some could receive no such penalties whatsoever. 

I think it's significant what Fr. Fenton writes in his article on the weight of the theological manuals:

http://www.catholicapologetics.info/modernproblems/vatican2/Manuals.htm


"What seems to displease Father Baum is the fact that the unanimous teaching of the scholastic theologians in any area relating to faith or morals is the teaching of the ordinary and universal magisterium of the Church. The manuals, like those to which we have referred, are books actually used in the instruction of candidates for the priesthood. They are written by men who actually teach in the Church's own approved schools, under the direction of the Catholic hierarchy, and ultimately, through the activity of the Congregation of Seminaries and Universities, under the direction of the Sovereign Pontiff himself. The common or morally unanimous teaching of the manuals in this field is definitely a part of Catholic doctrine.
It is quite obvious that the individual opinions of individual authors do not constitute Catholic doctrine, and could not be set forth as such. But there is a fund of common teaching (like that which tells us that there are truths which the Church proposes to us as revealed by God, and which are not contained in any way within the inspired books of Holy Scripture), which is the unanimous doctrine of the manuals, and which is the doctrine of the Catholic Church. The unanimous teaching of the scholastic theologians has always been recognized as a norm of Catholic doctrine. It is unfortunate that today there should be some attempt to mislead people into imagining that it has ceased to be such a norm in the twentieth century."

Of course YOU think it's "significant" ... because you imagine that it backs your support of BoD.  Yeah, that's what Bellarmine thought too about the Augustinian problem cited above (did you even bother reading it?).  It's also significant that this "Father Baum" also disagrees with Fenton's hyperbole.  Theologians are not part of the Ecclesia Docens and enjoy only the same infallibility as the Ecclesia Credens.  Indeed the Ecclesia Credens cannot defect when it comes to dogmatic truth, but there's nothing to prevent an erroneous opinion from becoming widespread.

So, do you accept the teachings of Vatican II?  In that case, it was the bishops of the world -- who actually ARE the Ecclesia Docens -- who taught Vatican II to the Church.    So those things taught by the Ecclesia Docens to the Church in Ecumenical Council can be rejected by you as not being part of Catholic doctrine while the theology manuals must be accepted as such?

That’s an issue I am thinking about, because Clearly there is an implied rift between the Fathers ending with St. Bernard and the Scholastics Beginning with Hugh and Richard of St. Victor. If this were consistent, then we would have to say that at some point the scholastics themselves seem to be the radical freethinking innovators in their deviations from St. Augustine and the Augustinian fathers.

Interestingly enough Cardinal Ratzinger seemed to suggest this when he wrote an entire document on the duties of the Theologian and says the consensus of bishops are to be understood in a “diachronic” (“Throughout Time”) sense and not merely “Synchronic” (Simultaneous at a given moment). This really puts tradition back in the driver seat. BUT then how are we to understand the claim that a unanimous consensus of the scholastics is at least infallibly safe and certain? I assume it would be more nuanced than flat out vulgar rejection.

A contradiction between the view of St. Augustine as to the fate of unbaptized infants and that of St. Thomas and those who asserted a Limbo of "natural happiness" seems to depend on the correction translation of the Council of Constance Canon 3.

No, that's simply not true.  Bellarmine and the scholastics were all keenly aware of the contradiction and there were two competing schools of thought on this matter as a result.  Read the CE article.  Either that or Bellarmine and St. Thomas were just idiots who didn't understand St. Augustine properly.

From “A Manual of Catholic Theology” 1906

http://www.strobertbellarmine.net/wilhelm_scannell_04.html

“The consent of Theologians produces certainty that a doctrine is Catholic truth only when on the one hand the doctrine is proposed as absolutely certain, and on the other and the consent is universal and constant (Consensus universalis et constans non solurn opinionis sed firmae et ratae sententiae). If all agree that a particular doctrine is a Catholic dogma and that to deny it is heresy, then that doctrine is certainly a dogma. If they agree that a doctrine cannot be denied without injuring Catholic truth, and that such denial is deserving of censure, this again is a sure proof that the doctrine is in some way a Catholic doctrine. If, again, they agree in declaring that a doctrine is sufficiently certain and demonstrated, their consent is not indeed a formal proof of the Catholic character of the doctrine, nevertheless the existence of the consent shows that the doctrine belongs to the mind of the Church (catholicus intellectus), and that consequently its denial would incur the censure of rashness.”

Again, we are called upon to adhere to the unanimous consensus of theologians in regard to their theological notes.

We don’t have the right to be rash in denying what the theologians say is certain. It’s a mortal sin. You have to work around this positively to be without mortal sin.

Indeed, when Vatican I defined the OUM, it referred to teaching something "as divinely revealed" ... so there's an separate note of infallibility involved.  See the passages I bolded in your quotation above.  BoD falls into the category of the final bolded passage.

Again, Bellarmine believed the same thing regarding the Augustinian position on infants who die unbaptized ... and yet rash theologians like Abelard and eventually St. Thomas corrected the problem.  So I plead guilty to "rashness" here, the rashness of a St. Thomas.