OP is saying there is a "great Catholic truth" not being taught. I am asking where has the Church ever taught this in the past? If the Catholic Church has never taught this, well...then there is no truth...because of the fact that the Catholic Church already has the truth. Nothing is being added to it.
It follows, therefore, that the OP is merely a speculation based upon no theological grounds, whatsoever.
I cannot quote you "chapter and verse" at the present time for lack of my own personal time. However, I will say this for the time being, the OP is not "merely a speculation based upon no theological grounds,
whatsoever."
On the other hand if you have any clear theological grounds to refute what I stated in my OP, I would ask you to present them.
Also, I will leave you for the time being with the following material to ponder:
"The practice of offering the holy Mass for the repose of the soul of the deceased originates in the early church. From the beginning the church has honored the memory of the dead and offered prayers in
suffrage [i.e., intercession] for them, above all the Holy Sacrifice of the Mass, so that they may attain the beatific vision of God” . How or if God intercedes is determined by God, not by us. There is nothing in Church teaching which asserts that God is incapable of interceding in the way I stated in my OP. After all He is omnipotent
and what I asserted in my OP does not contradict any truth of Church teaching. God, of course, can not contradict the truth including the infallible truth of His Holy Church which in no place excludes the power of God to act as I asserted when one prays and sacrifices for a deceased soul as I spoke of in my OP. If you have proof contrary to this I would ask that you present it.
Praying for the repose of the souls of the deceased is rooted in the Old Testament. Judas Maccabees offered prayers and sacrifices for the Jєωιѕн soldiers who had died wearing pagan amulets, which were forbidden by the Torah. II Maccabees reads, “Turning to supplication, they prayed that the sinful deed might be fully blotted out” (12:43). Continuing, “(Judas Maccabees) took up a collection among all his soldiers, … which he sent to Jerusalem to provide for an expiatory sacrifice. In doing this he acted in a very excellent and noble way, inasmuch as he had the resurrection of the dead in view; for if he were not expecting the fallen to rise again, it would have been useless and foolish to pray for them in death. But if he did this with a view to the splendid reward that awaits those who had gone to rest in godliness, it was a holy and pious thought. Thus, he made atonement for the dead that they might be freed from sin” (12:46).