Can anyone suggest a way that might let me recover from this error?
Have you checked the drive and file system on the NAS? It could be hardware failure.
For whatever reason, I'm pretty sure it will take my hardware days or more likely, months, to re-download the blockchain.
If you have enough disk space, you can just try a new download. If you put
chainstate on a local SSD, set dbcache to 4096, have enough RAM (at least 8 GB but more is better of course), and have enough bandwidth, any modern computer should be able to download the Bitcoin blockchain within a day.