Charges against Vader were stayed not withdrawn, so he's not totally off the hook. AFAIK, there has never been any indication that LE has other suspects. IMO, not really a "Whoops" as much as they couldn't meet the threshold and are hoping the next year will provide what they need to do so. Another case without bodies being recovered.
Yes and in cases like this -- where they don't have enough evidence or the wrong person -- an announcement is made.
Police don't just say they're not looking for other suspects and then secretly start looking for other murder suspects, as it was suggested they could be doing in this case.