I am assuming that all the errors were on datasets which no longer exist and running the scrub tidied this up. All the datasets I wanted to retain never had permanent errors.
The pool is functioning perfectly okay and now that I have reverted to 11.1, I have not had any hardware/software issues whatsoever. Previously it would virtually never complete a backup from either of my Windows boxes, even incremental backups and particularly from one box backing up over wifi. They have now gone through several days of error-free backups.
Rather than wait a couple of weeks to know for sure, I have kicked another scrub off manually.