In the Arrancar\Hueco Mundo arc she healed Grimmjaw's hand and bought back a hollow girl killed by Grimmjaw's cero. Can she bring back the dead Shinigami in the 1000 year war arc??
My bet is Most likely yes, she can reject phenomena not heal so yes it's possible.
But I dont think that's how the people who died in the blood war are going to come back, I think they will come back but not by having Orihime heal them
That's not healing. If I recall, Orihime has the ability to reverse whatever phenomena that happened. Even the ones she didn't witness, she can negate whatever happens to who she intends to that power on. So, I don't see why she couldn't.
It isn't healing. It is the ability to reverse any phenomena that has occurred to a specific subject. If I lost my arm, she would reverse the actual occurrence, to a point where it is unnoticeable that my arm was ever severed in the first place. While it is similar to healing, a more accurate description would be "rejection" and/or "reversal".
Here, her ability is shown to be able to reject and even reverse death, though I think this ability can only be used if the subject has only been recently killed.
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Given that it's been at least a day or two since Yhwach's second invasion, and at least a week since the first, I doubt the death of the Shinigami at the hands of the Sternritter and Quincy will be reversible. Though that's assuming her ability to reverse death can only be done within a certain time frame before the process of revival becomes impossible.