Sorry if this is too speculative, let me know if against the rules.
But after hearing that ‘remains’ have been found (I.e. not a body), I wonder if WC burnt body, therefore potentially destroying physical evidence in the event that her remains were found. If body is in such a bad condition then police may not be able to determine cause of death or whether she was sexually assaulted?
Could leave him open to trying to claim that it was a RTA or other accidental situation and he panicked, disposing of the body / evidence rather than reporting the accident for fear of his career and reputation
The killer of April Jones tried this, but was convicted of murder anyway. There was no body at all in that case.
I suppose that assuming WC is the assailant, if the body is badly damaged (which it might not be) he could try to wriggle out of it but it would need to be a hell of a story. Whether it would succeed would depend on what evidence there is which we don’t know about - eg on the last CCTV sighting, whether they found any blood at the scene of abduction/accident which is consistent with his story.