Can't really remember that... also why would Nergal sustain the MC?
It's not like they fused their essence or are bound by soul... the familiar lives in the meta world which the Protal network connects to, she only had to be summoned by the MC, that however requires a strong enough magical pressence... Nergal lend his pressence to the MC to call for his familiar, she later evolves based on the MCs preferrences as familiars try their best to bind themselfes to a powerful master that sustains them, his children are unbound and need to live in the Meta world until the MC builds a focus point in the Tower that allows them to remain there until they are old enough to find themselves their own masters or mistresses.
Zivah was alive dureing the war and wittnessed the chaos and destruction, though there is no mentioning of her being an active combatant, her low Level and lack of even halfway decent equipment are a pretty goood indicator aswell.
She had a jump on the distracted and still green Peasant, a knife on his throat and the thread of being cursed for eternity should he attempt anything stupid.
Healing haveing a detrimental effect in fantasy lore is soley for the undead, and as far as I recall the Wraith in the Cult of the Death hideout were vurneable to healing aswell (not sure about the Zombies and thralls in the graveyard).