Don't know the celeb but Gimple was very forthcoming about the upcoming season without spoiling things much.
Nathan Fillion. Firefly? Lost? Castle? He is a nerd celeb god, right up there with Bruce Campbell. Awesome dude.
I guess it could be argued that Crazy Clara was not real, but it doesn't fit the storyline IMO. Crazy Rick is gone. That was symbolized by the whole metaphor with Lori not appearing at the end of last season when they brought the new people in. The idea I think is that he is being slowly healed by a spirit of community. I don't see how it serves to move the plot forward if he starts going nuts again.
I think it's more likely she was real and had simply survived by virtue of having a "pet" around like Michonne did. And I don't think Rick took anything because what did she have of value? That knife? I'm sure that tent and those sleeping bags reeked of stank. They could easily get all that stuff from Big Lots anyway.
Someone should be working on digging some tunnels out of that place too. Just to be prepared.