"Aren't the newborn (along with righteous pagans, etc.) destined to heaven through the operation of invincible ignorance?"
Perhaps you misspoke, which seems the simplest explanation for your later posts which don't seem to share this quite mistaken thought.
We must be careful not to veer into thinking as if Christ Himself is bound by His Sacraments.
Yet we also must be careful not to veer into thinking that He will not keep His promises. "Amen, amen, I say to thee, unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God." "Amen, amen, I say unto you: except you eat the flesh of the Son of man and drink his blood, you shall not have life in you."
The truth is that they go to Limbus Infantium, which is the state of perfect natural happiness. They are not in a state of mortal sin, as was alleged here and are not condemned to suffering.
I will not re-argue about the Limbo on this thread when I presented the teaching of the Catholic Church on the subject on a dedicated thread: Limbo