The thing about Dummies is this: usually, they're used if the story is told from the POV of a Sue/Stu. That way, the author wraith possesses the Dummy and the agents are safe to do their work as usual.

I'm not sure how that would work if it was from the POV of a canon character who isn't replaced (according to you). It is perfectly possible for a wraith to possess an agent without a dummy, though: Aster wrote a mission sporking a fic that was in first person where the author wraith possessed the central character of the continuum the story was set in.

I'm not going to make a judgment call, but I'm not sure you need the Dummy in the first place. Dummies are only there to keep the agents from getting Sued by a Sue in a first person fanfic. If it's a different character altogether, then the wraith probably won't need a new body. I'm not going to make a judgment call, but in your place that's what I'd keep in mind.

I haven't used a Crash Dummy (yet), but I would think that if one is in use, the real character is stuffed in a plothole somewhere. Every mission I've seen with a Dummy, there's a Sue-wraith animating the thing.