Rising from death, and defeating sin by rising to save humankind is not the same as dying to save the universe as you describe. I also seriously doubt that she did it purely as a selfless act. To think she did it to be selfless goes against everything we have seen from the Denarians to this point.
She told Dresden that she was trying to save the universe. I think she was unusually candid in her talk with Dresden, revealing herself a bit. She was on the cusp of ultimate sacrifice and so, I think, open. Her selfless sacrifice was for the greater good as she saw it. There are plenty of stories about redemption and change. Why can’t Deirdre’s be one? I don’t actually think this mantle acquisition will happen, but it is parallel adjacent to the myth and it’s not impossible that this sort of selfless sacrifice for the universe is how the jesus mantle is acquired.
And, it could be a fun twist if Deirdre came back as the jesus mantle holder. That would put Nic in the role of the father figure in the trinity. Plenty of posters here predict some sort of redemption for Nic, so why not? It’s not like the mythical figures who have held roles in this construct are fluffy bunny good creatures, after all. They did plenty of petty stuff, wholesale slaughter and nastiness.
Also, keep in mind this is all taking place in a fantasy universe where all this myth and magic is real, not our universe.