I'm not entirely convinced that event_type_id = 11 is correct above; at least it wasn't for me.

You did however point me in the direction of a solution. Thank you!

The code that solved the problem for me was the following:

This has removed the spurious unread marks in the conversations application. Success! However, it's not corrected the unread counts within the Conversations Inbox desktop widget which seems to be getting its unread marks from elsewhere. I'll try rebooting to see whether that helps; removing and readding the widget did not.