Adaar and Gabriel were supposed to test run letting Lucifer possess her instead, but we don't have time and now that he knows about the Fade shit, he won't bite. I think our best bet now is to open the cage's door and get Cas to expel Lucifer on his own.
I have no fucking idea if that's possible, but Gabriel's with us, maybe if he and Lucifer scrap for a while Lucifer will take a beating and Cas will have an edge.