i suggest you open a case with NetApp to try and find why the snapshot got stuck.
they should easily search in the syslog and snapmirorr.log for the snapshot name and figure out their history (you can also do it yourself in Active IQ. but they can maybe correlate it to a bug based on internal database faster).
once it found they could give you a full action plan for recovery. including when to force the deletions and what should be done before / after.