Limbo can't be applied to adults because the adults have reached the age of reason and sinned. Besides, I have never heard of unbaptized adults going to Limbo, which is an earthly paradise, you'll have to provide sources for that idea. 

There are no sources; it's speculative.  Limbo itself began as speculation.

I can see cases where someone has lived a naturally-virtuous life ... and then performed some naturally-heroic act like giving his life for others where, although he did not have supernatural charity, the natural virtue would offset the natural (temporal) punishment due to various sins he may have committed during his life, thus wiping the slate clean on the temporal side.

Speculations on this matter on a public forum is not a good idea, before you know it, every non-Catholic is in Limbo of the Infants. It is called Limbo of the infants for a reason.

LT, you’re missing the point.  Apart from the “Limbo of the Just” in the OT, the entire idea of Limbo for the New Testament is speculative.  I don’t have to believe any of it.  It’s not defined. So no one can say the details 100%.

The Limbus
Infantium, or Limbo of Infants, is the abode where the souls of those who die in Original Sin, but without personal
(actual) sin, are deprived of the happiness that
would come to them in the supernatural order,
but not of the happiness of the natural order.       It is an article of faith,
most recently confirmed at the dogmatic Council
of Trent, that those who die without Baptism, and in whose case the want of Baptism has not been supplied in any other way,
cannot enter heaven.  Nothing imperfect
can be in the presence of God, as we know from the Apocalypse:  "There shall not enter into it [the
glory of God] any thing defiled"
(21:27/DRV).  The great majority of the 

authoritative theologians of the
Church, among them Peter Abelard, St. Bonaventure, St. Thomas Aquinas,

Duns Scotus, teach that infants dying in Original Sin suffer no "pain of sense," but are excluded from heaven.

This opinion is no modern invention, for it is found in St. Gregory of nαzιanzus
(Or. in Sanct. Baptism 23, PG XXXVI:389), one of the Great Eastern Fathers of the
Church.  St. Thomas Aquinas teaches that such souls do not suffer pain of
sense because pain of punishment is proportioned to personal guilt, which does
not exist here.  He says that those in limbo do not grieve because they
cannot see God any more than a bird grieves because it cannot be a king.
 "No, they rejoice because they share in God's goodness and in many
natural perfections," he says.  The unbaptized in limbo know and love
God by

 the use of their natural powers, and have full natural happiness.  



(De Malo, 5:3; Sent. II
d. 33 Q. 2 A. 2)

        St. Thomas Aquinas, the Church's principal theologian, teaches that 

unbaptized children do not suffer pain because of their privation.  They are 

not capable of the grace of the supernatural order, which is not owed to man 

(the word "grace" itself denotes something "gratuitous" from God), but 

possess a natural well-being that results from their being united to God by 

their participation in His natural goods.

        Following the teachings of the Prince of Theologians, St. Augustine 

of Hippo, Pope St. Gregory the Great, and the Scholastic Theologians, 

including the Seraphic Doctor St. Bonaventure and the Universal Doctor St. 

Thomas Aquinas, the Seventeenth Oecuмenical Council in 1438-1445) adopted an 

canonized as a matter of faith:  "illorum animas, qui in actuali mortali 

peccato vel solo originali decedunt, mox in infernum descendere, poenis tamen 

disparibus puniendas [the souls of those who die in actual Mortal Sin or only 

Original Sin, thereupon descend into Hell, but to be punished with disparate 

punishments]."

        In 1794 Pope Pius XI confirmed the existence of Limbo as a place 

lacking the Beatific Vision, but without the pain of punishment

This is the teaching of the Church and cannot be denied, having behind it 

both antiquity from Patristic times and from the Scholastic Theologians, 

including St. Thomas Aquinas.

So you argue that there is no salvation without water baptism, but there is forgiveness of sin without baptism, but not enough forgiveness as to grant salvation?

From reviewing your citations of Church teaching, it seems certain that unbaptized infants do not suffer any pain of sense, but not completely certain whether they suffer pain of loss.

Here I agree with St. Thomas Aquinas that there's no pain of loss.  Human nature is not deprived by virtue of not having the beatific vision since it is not within the capacity of human nature to experience this.  I like the example of a bird not grieving on account of not being a king.

 

Before all decision to create the world, the infinite knowledge of God presents to Him all the graces, and different series of graces, which He can prepare for each soul, along with the consent or refusal which would follow in each circuмstance, and that in millions of possible combinations ... Thus, for each man in particular there are in the thought of God, limitless possible histories, some histories of virtue and salvation, others of crime and damnation; and God will be free in choosing such a world, such a series of graces, and in determining the future history and final destiny of each soul. And this is precisely what He does when among all possible worlds, by an absolutely free act, he decides to realize the actual world with all the circuмstances of its historic evolutions, with all the graces which in fact have been and will be distributed until the end of the world, and consequently with all the elect and all the reprobate who God foresaw would be in it if de facto He created it." (The Catholic Encyclopedia Appleton, 1909, on Augustine, pg 97)

From Mystical City of God , by Sister Mary of Agreda.
 
537. Seeing him (Judas) thus beside himself Lucifer inspired him with the thought of hunting up the priests, returning to them the money and confessing his sin. This Judas hastened to do, and he loudly shouted at them those words: "I have sinned, betraying innocent blood!" (Matth. 27, 4). But they, not less hardened, answered that he should have seen to that before. The intention of the demon was to hinder the death of Christ if possible, for reasons already given and yet to be given (No. 419). This repulse of the priests, so full of impious cruelty, took away all hope from Judas and he persuaded himself that it was impossible to hinder the death of his Master. So thought also the demon, although later on he made more efforts to forestall it through Pilate. But as Judas could be of no more use to him for his purpose, he augmented his distress and despair, persuading him that in order to avoid severer punishments he must end his life. Judas yielded to this terrible deceit, and rushing forth from the city, hung himself on a dried-out figtree (Matth. 27, 5). Thus he that was the murderer of his Creator, became also his own murderer. This happened on Friday at twelve o'clock, three hours before our Savior died. It was not becoming that his death and the consummation of our Redemption should coincide too closely with the execrable end of the traitorous disciple, who hated him with fiercest malice.
 
538. The demons at once took possession of the soul of Judas and brought it down to hell. His entrails burst from the body hanging upon the tree (Acts 1, 18). All that saw this stupendous pimishment of the perfidious and malicious disciple for his treason, were filled with astonishment and dread. The body remained hanging by the neck for three days, exposed to the view of the public. During that time the Jews attempted to take it down f rom the tree and to bury it in secret, for it was a sight apt to cause great confusion to the pharisees and priests, who could not refute such a testimony of his wickedness. But no efforts of theirs sufficed to drag or separate the body from its position on the tree until three days had passed, when, according to the dispensation of divine justice, the demons themselves snatched the body from the tree and brought it to his soul, in order that both might suffer eternal punishment in the profoundest abyss of hell. Since what I have been made to know of the pains and chastisements of Judas, is worthy of fear-inspiring attention, I will according to command reveal what has been shown me concerning it. Among the obscure caverns of the infernal prisons was a very large one, arranged for more horrible chastisements than the others, and which was still unoccupied; for the demons had been unable to cast any soul into it, although their cruelty had induced them to attempt it many times from the time of Cain unto that day. All hell had remained astonished at the failure of these attempts, being entirely ignorant of the mystery, until the arrival of the soul of Judas, which they readily succeeded in hurling and burying in this prison never before occupied by any of the damned. The secret of it was, that this cavern of greater torments and fiercer fires of hell, from the creation of the world, had been destined for those, who, after having received Baptism, would damn themselves by the neglect of the Sacraments, the doctrines, the Passion and Death of the Savior, and the intercession of his most holy Mother. As Judas had been the first one who had so signally participated in these blessings, and as he had so fearfully misused them, he was also the first to suffer the torments of this place, prepared for him and his imitators and followers.
 
539. This mystery I was commanded to reveal more particularly for a dreadful warning to all Christians, and especially to the priests, prelates and religious, who are accustomed to treat with more familiarity the body and blood of Christ our Lord, and who, by their office and state are his closer friends. In order to avoid blame I would like to find words and expressions sufficiently strong to make an impression on our unfeeling obduracy, so that we all may take a salutary warning and be filled with the fear of the punishments awaiting all bad Christians according to the station each one of us occupies. The demons torment Judas with inexpressible cruelty, because he persisted in the betrayal of his Master, by whose Passion and Death they were vanquished and despoiled of the possession of the world. The wrath which they had conceived against the Savior and his blessed Mother, they wreck, as far as is allowed them, on all those who imitate the traitorous disciple and who follow him in his contempt of the evangelical law, of the Sacraments and of the fruits of the Redemption. And in this the demons are but executing just punishment on those members of the mystical body of Christ, who have severed their connection with its head Christ, and who have voluntarily drifted away and delivered themselves over to the accursed hate and implacable fury of his enemies. As the instruments of divine justice they chastise the redeemed for their ingratitude toward their Redeemer. Let the children of the Church consider well this truth, for it cannot fail to move their hearts and induce them to evade such a lamentable fate.

Even one of your own quotes is general enough to support the possibility that an adult can be in limbo.  I don’t get why you’re drawing a line in the sand on this. It’s unknown, but you’re acting like it’s obvious.  It’s debatable.  
.
.
[Those dying with only original sin on their souls will suffer] no other pain, whether from material fire or from the worm of conscience, except the pain of being deprived forever of the vision of God."
-Pope Innocent III (1160-1216), Corp. Juris, Decret. l. III, tit. xlii, c. iii -- Majores