Bud "Incarnation" is a synonym of manifestation and is a far more accurate way to describe a god "manifesting directly", Incarnation literally means "embodied in flesh or taking on flesh" while Manifestation includes less "real" acts like possession, dreams & visions.I think she's not the incarnation of quetesh, but Quetesh herself, and unlike Nergal she has worshipers and enough power to manifest directly
there's a chance that nergal being the god of death, destruction etc is barren and couldn;t knock quetesh up.
My personal favourite theory of Nergal/Qetesh is that they are twins... you can't have light without dark... they define each other, I'll go so far as to suggest they could be the same entity (like Janus or some other schizophrenic deity).