Eva had assumed the guise of an Archangel among a thousand legions of Archangels charged with supervising a quadrant of reality that was too close to the Primordial Domain of Light than she was comfortable with. Still, orders were orders, and Rowan had never led her wrong.
She could have taken the form of a Sovereign or a Power if she wanted, but being an Archangel was a strict requirement from Rowan for this mission. She was not the tip of the spear, her role was more of an overseer.