Way I see it, the only expendable character is Martha. Also, her death - while traumatic for Castle, may also serve to bring him and Beckett together, what with her having lost her mother previously.
Also, Pi= 3.14/3XIV turn the V on its side and you have 3XK.
I think it could be his father. It would kind of make sense to have it be the opposite for him.
Good catch.