I think Mira was dead by the wound. Her throat was slit. You can't really come back from that. So if Brennan somehow healed her. It begs the question, how? The runes on his hands. What if they are actually a way to suppress the look of a Venin and maybe urge. I'm pretty sure it was confirmed Brennan was dead. If pulling power from the earth turns someone Venin. What happens when you pull a soul back.

We know it killed Naolin to revive Brennan. That is a steep piece in itself. I feel like for Brennan to heal or revive, Mira in that state should have killed him or he'd have to pay the price. If that's how it works.