Nyx can't "resurrect" people unless they have her mark. I don't think she actually resurects them - it's more like telling to the Death "fuck no! he/she stays here!" when someone marked gets killed. She has powers of Death (necromancy?) on her side and she isn't some kind of words greater healer.
Nyx also brought him back in lvl3, which can suggests that this is more like "loading a quicksave" she makes when putting a mark or that evolution between superhuman levels is process that can be stopped but then the superhuman get send to the moment before it started.
There are a few indications that make me think this isn't exactly the case.
1. Oscar was never marked, but Ella says Nyx is capable of helping Charlie ressurect him. Could be a lie but somehow I doubt it.
2. If it's a quickload, then it means she has to constantly reapply the marks. What do you think would happen in this scenario:
Nyx marks a Superhuman.
The Superhuman evolves normally.
After a month, The Superhuman fights a battle and dies
Nyx ressurects the Superhuman
What would the Superhuman's level be?
I mean sure, it's possible that the Superhuman would have to be ressurected at a lower level, but if I had to guess, I'd say that this isn't the case. Her relationship with the stored souls seems more holistic to me and the reason is that the Souls inside her river have feelings about her. There is a certain progression which indicates that time doesn't suddenly stop for them.