I have merged the users - strangely the resulting user was activated but not approved, and the approve button was grayed out, so I couldn’t just approve them.
Following the workaround described here, I deactivated then reactivated the account, and was then able to approve.
I reran all 3 queries, and (in reverse order) am showing 0 users in the sign up queue, 946 unapproved users without an associated reviewable object, and ‘zero results’ for unapproved but active users.