how to clean the snapshot mess?

have you tried canceling session between STD and VDEV ? I had a similar issue with BCVs ..ended up calling support as i could not manipulate meta members (some were showing split and some were synchronized)

Sometime it happens .. drives that fail usually give impact only on some meta members .. but we (since I work with the same tools that customers use) are allowed to manipulate only whole metas, as you already noted ...

Usually I try to add all the available -force (included the -whatever_you_can_force option and -may_the_force-be-with-you ) .. But however I have to call the support and open a SR (if the box didn't already opened one) requesting someone to clean the mess

Sorry .. that's the way metas work