Do the conditions for mortal sin apply to non-catholics? There are three conditions for a sin to be mortal:


It must be a grave sin. Eg. Anything from the 10 commandments, Adultery, Murder etc.
It must be committed with full consent of the will.
The person who commits the sin must be aware that it is a grave sin. ("Full knowledge")


This leads me to wonder, are non-Catholics who don't believe in mortal sin by that very fact, immune from committing mortal sins? My reasoning is that, a person who doesn't believe in mortal sin could be said to be "unaware" that their sins are grave, and if they are "unaware" that their sins are grave, then their sins are not mortal, therefore they won't go to Hell (I think the correct terminology is to say that they are in invincible ignorance).

Extrapolating this line of reasoning, doesn't this mean that most if not all non-Catholics will be saved? Because these non-Catholics are presumably "unaware" that they have to be part of the Catholic church in order to be saved. And if they are "unaware" of this then they are not in a state of mortal sin because they don't have full knowledge and are thus in invincible ignorance and wont go to Hell.

On the other hand, imagine that the non-Catholic has only met a single Catholic in their entire life and the only thing that the Catholic said to them was "you have to convert to my religion or you will go to Hell". In this case is the non-Catholic still "unaware" that they need to join the church or does a brief warning such as this count as being given "Full knowledge"?

Finally, it seems possible to say that all non-Catholics are in a state of invincible ignorance, because if they had full knowledge that they have to become Catholic to be saved, then they would hurry up and convert. But they don't do this, so it seems fair to assume that they are unconvinced and therefore unaware. (Who on earth would become sincerely convinced that to go to Heaven and avoid Hell they have to join the Church, and then not do it?)

In summary my questions are 


How is it possible for a non-Catholic to commit a mortal sin? and
Are all non-Catholics in the state of invincible ignorance as per my reasoning above, and if not, where did I go wrong?
If the non-Catholic is not baptized, the non-Catholic has original sin and is damned, "unless before the end of life the same have been added to the flock," as Pope Eugene IV puts it in his Cantate Domino.

If the non-Catholic is baptized, he does not have original sin, but he can certainly commit actual sins (mortal or venial).

Cain and Onan—who committed the sins of fratricide and contraception, respectively—were before Christ, yet that did not except them from sin.