I reject the premise that one can't sin through ignorance as the condemned statement of Abelard proves and I also would like to point out that there are many ways to commit mortal sin.
The first option is definitely an acceptable position even if the second is incorrect, which is possible.
What isn't possible is for a non-Catholic to go to heaven because every human creature by absolute necessity must be subject to the Roman Pontiff, possess the Catholic faith and be inside the Church.
I'm looking forward to seeing your answer tomorrow.
Abelard did not specify mortal sin, which is impossible in a state of ignorance (either that, or all the manuals have been wrong about how one commits a mortal sin for a millenium, and we're all damned). Venial sin does not rob the soul of grace, but that is all one ignorant can do. So the Abelard quote does not help you, as previously mentioned.
So we return yet again to the question:
If one is invincibly ignorant, by what means is grace forfeited (since full knowledge is necessary for the grave sin by which the grace is lost)?
Conversely, if the grace is not forfeited, how would such a one be damned?