You still haven't defined at what point something becomes in your eyes irredeemable. I apologize for asking again, but I don't see that we can have a productive discussion without being aware of the point where you believe that occurs. Of course it's largely a judgement call, but without knowing your criteria for this matter... we can't effectively try to demonstrate that Liches are or are not redeemable.
I'm not familiar with Exalted, but I would argue that even then, there is no such thing as an irredeemable character, in the end, and if White Wolf suggests that there is, I would argue that it's poor writing and not very good vis a vis their own typically level headed stance on morality. But again, I'm not familiar with why the character would be irredeemable.
But again, it isn't really possible for us to present constructive points without having a better idea of where redemption is no longer possible in your eyes. We'll say in D&D or in Roleplaying games in general (and if the rules are different for those then that helps too) because otherwise formulating arguments against or for this becomes something that is impossible for us to do.