Do perilunate dislocations also occur in isolation (i.e without associated scaphoid fractures)? If so, how often?
Yes they do, and in ~40% of cases no scaphoid fracture is present.
What is the relationship of the lunate to capitate and distal radius in these injuries?
The radiolunate articulation is preserved, whereas the capitate dislocates almost always dorsal to the lunate.
Fracture of the scaphoid with dorsal dislocation of the carpus relative to the lunate.