From the CCC:
633 Scripture calls the abode of the dead, to which the dead Christ went down, “hell” — Sheol in Hebrew or Hades in Greek—because those who are there are deprived of the vision of God. Such is the case for all the dead, whether evil or righteous, while they await the redeemer: which does not mean that their lot is identical, as Jesus shows through the parable of the poor man Lazarus who was received into “Abraham’s bosom” “It is precisely these holy souls, who awaited their Savior in Abraham’s bosom, whom Christ the Lord delivered when he descended into hell.” Jesus did not descend into hell to deliver the damned, nor to destroy the hell of damnation, but to free the just who had gone before him.
“The Just” are those who followed the Mosaic Law before the coming of Christ.
As for those who were ignorant of the Law:
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law.
Given this, I do not find it impossible to hope that those ignorant of the Law were subjected to purification, such as purgatory, “so as to achieve the holiness necessary to enter the joy of heaven.”
Though, I should say that as to you’re supposition that the “fact of his preaching in hell would demonstrate his true nature, and every inhabitant of hades would naturally accept him?”
The fallen angels rejected God even though they knew his true nature. Perhaps there were some souls who died outside of the law who still rejected the possibility of heaven even though they were given the chance.