I find that to critique this argument, we must look at each verse you produced and look at the context. Eric has already provided a perfect example of this “differentiation” of sin with the mentioned verse. Now, let us move to the critique.
With Romans 6:23. we can both agree that yes, sin does wage death. This death obviously refers to the Second Death, which is Hell. We can both agree upon this. However, this statement is not referring to whether there is mortal sin or venial sin. It is making a statement referring to how we are saved through the grace of our Lord, which is true. Therefore, I find no reason to assume that this verse is indicative of defining whether there is mortal or venial sin.
Ezekiel 18:20, when taken into context, is not trying to define whether there is mortal sin. Within the context, we find that Ezekiel is referring to the idea that the personal sins of the Father would be passed to the Son, and that this idea is wrong. Each individual is responsible for the personal sins that they commit. This is the message of this verse within the context surrounding it. However, we can find an implicit understanding here, with a differentiation of sin (although not of the mortal/venial aspect). We agree that Original Sin is stained upon all humans at birth (although we may disagree about the Blessed Mother, who did not recieve this stain). So, is this verse referring to Original Sin? Of course not! We all agree that Original Sin is passed down through generation to generation. Therefore, this must be in reference to personal sin. This is an example of a differentiation of sin. There are two general kinds: Original and Personal. Therefore, we can see that you would be mistaken, there is already an implicit statement made that there is a distinction between sin, Original and Personal. It seems that I have gone off track. Back to the point, we see that this verse is removed from the context. We agree that the soul which sins will die. However, do we not all sin? Do we not find ourselves a spot which is condemed by Scripture if we claim that we do not sin? Look upon this:
[BIBLEDRB]1 John 1:7-9[/BIBLEDRB]
It is clear that we all sin. Now, we can rightly assume that if we all sin, then do we agree that rather than being the “soul which sins will die”, it is that the “soul which sins and does not repent, and remains unrepentant will die”? Therefore, knowing that this verse in Ezekiel is taken out of context, we find that it cannot be used for defending the idea that there is no differentiation of sin, for we can already see one (Original and Personal).
Now, with James 2:10-11, we see again another example of a removal of context. Why is James speaking of this transgression of Law? We see that it is in response to those who believed that they could transgress the Law at one point, but remain pure because they avoided transgressing a part of the Law. Earlier in this chaper, we see that those who keep the Royal Law of loving your neighbor as yourself are commended. This verse is, again, not about making any differentiation between sin. That is not the purpose of this verse. Rather, it is correcting those who assumed that they could keep a partiality to the Law. This, again, is taking a verse out of context.
Ultimately, we find that even our Lord Jesus spoke about this. Look at this:
Why is there a difference between the servant who knowingly did what was wrong, and recieved the just punishment, but the servant who did not know that he was doing what was wrong, was beaten lightly? Why is this such a clear differentiation? Compare this to the understanding of mortal and venial sin. Mortal sin is committed when a person knowingly commits a sin with full intent, whereas venial sin is where a person commits a sin without full knowledge or consent (there are other factors to mortal and venial sin, but I’m just listing off what I remember). This teaching by the Lord reflects exactly what the Church teaches. I believe this is a perfect article to check out: