One of the biggest mysteries called into question is where Sage’s healing power draws its life. Omen asks this question himself “The life you give, do you ever wonder where it’s taken from”. My theory is that Sage is actually taking life from the other copies in the parallel Earths that were created (or found?) during the First Light event.

To start with, we would have to subscribe to the theory that there’s only really one Omen split across all realities. This is based off his interactions with his copy not referring to them as a copy “There is another of me, how many times did I get ripped apart, how many times did I die?”

With that theory in hand we move to Viper. Viper cares about Omen “Keep being their nightmare, old friend” but seemingly withholds knowledge from him about himself (Fred quote and “don’t die Sabine, I need your secrets”) and has likely worked with him in the past as lab partners “kills enemy Viper as Omen Experiment complete”. Why she withholds this knowledge isn’t really important to this theory, what is is she seemingly knows what happened to Omen.

Viper knowing what happened to Omen is vital for this last part, Viper’s interactions with Sage. They recently got a new voice line a week-ish ago “Sage: Viper, have you thought about my offer? You don’t have to- Viper: Never ever assume you can help me. You can’t help me, you can’t help them”. Now it could just be Viper lashing out, but if we take what she said literally here and make a somewhat supported assumption that “them” refers to the split-across-worlds Omen copies then why wouldn’t Sage be able to help him? She can heal on a whim and literally bring back people from the dead for what looks like the cost of radianite so why wouldn’t she be able to restore Omen?

My answer, given the mystery behind where she draws her power, is that she’s taking life from the copies of the other universes she’s fighting, maybe even without realizing it. This is partially supported by her voice lines about being weak, training hard to be better, then coming back strong enough “I wasn’t strong enough before, but now? Now I am strong enough for us all” which given this theory would mean she had to run a few missions without healing abilities, fight a few copies of her allies, then realize she could suddenly heal people and believe her training had “payed off”. We actually have not seen her heal anyone but her own allies (who she is simultaneously fighting) and since she keeps her power contained in expensive looking Jade orbs I’d imagine she probably hasn’t even tried healing anyone but her team yet and hasn’t realized she cant heal anyone but her team.

the reason this can’t help Omen is because he has no copies to take from. All the “copies” we see in game are just like-minded shards of the same Omen ripped across countless alternate realities so when Sage kills an Omen and heals her Omen she has only temporarily healed her Omen until his shard respawns itself (or he splits himself again probably via his ult) and places the “copy” back in the reality it was from. This would explain why she thought she could help Omen but Viper knows she can’t, because to her knowledge her power is working on Omen but any restoration she could do would be extremely brief and temporary. Through this theory we have effectively found the limit of Sage’s power. She cannot repair someone that is not within another universe and she has not absorbed and thus Omen cannot be restored to a human because he and his copies are not human in any universe. This would also explain how Sage can’t create life yet still be able to fully resurrect a deceased carcass no matter how much time has past (Cypher: So Sage, you can bring people back from the dead? No restraints? How very interesting).

tl;dr When Sage kills copies from other realities she is unknowingly taking their life to restore her own reality’s copies and this can’t help Omen because his “copies” are shards that regularly break off from each other so any restoration Sage could do is extremely temporary.