User blog comment:B214/Question and Answers 6/@comment-32724211-20170921081847/@comment-3285616-20170921084416

Fake Issei was something that Loki created, as he is the God of trickery, something like that would be easy.

As for Rias sleeping with Issei, no, I think it would only be with him. The same reason Issei was the one she asked to take her virginity to break of the engagement with RIser. It's not stated in the anime, but in the Light Novels, Rias herself states that she would not ask Kiba to do it as he is a pure knight. I think that Rias already knew Issei as a lecherous boy, and Rias is pretty pervy herself.