I was going to edit my post above but computer ate it up. Doh! So starting again a fresh. Hmm.
So IMO, KB will take on in a way Castle's role of filling in the story without having all the answers, etc. The faking the own death since it is similar to Derek Storm - I'm in the 3XK or CIA boat. Someone who has enough of a connection to Castle to be able to pull it off so it looks like he is fully involved in the whole thing, even though he isn't (hence why you mentioned Probable Cause).
Speaking of writers, any word who is writing 7.02? Since Hanning is doing 7.04 I thought it might have been TPW.