Diina playing the role of "Venus" is probably just so she can gather information from the demons, and she probably pulled that off, even though she is not a demon, using her memory manipulation skill.
Also, she probably is some sort of "Avatar" to the goddess, or the Goddess herself. And as for why she had to resort to spying If she is either one of those, It's probably because Efreet has a way to block her from prying with her God powers. And she probably chose the Seven Luminaries because higher up demons would either be immune to her memory manipulation, or would be able to recognize her even If she disguised herself. And that probably happens because, even though she is a Goddess (or an Avatar to her), she is probably not omnipotent.