Ok, good news: I managed to copy everything off the disabled disk8 while still in emulation mode to a drive off the unraid array.
Next, to fix disk9, I started in maintenance mode and from a terminal attempted:
xfs_repair -v /dev/md9
ERROR: The filesystem has valuable metadata changes in a log which needs to
be replayed. Mount the filesystem to replay the log, and unmount it before
re-running xfs_repair. If you are unable to mount the filesystem, then use
the -L option to destroy the log and attempt a repair.
Note that destroying the log may cause corruption -- please attempt a mount
of the filesystem before doing this.
I tried mounting and unmounting again with the same error, so back in maintenance mode I next tried:
xfs_repair -vL /dev/md9
ALERT: The filesystem has valuable metadata changes in a log which is being
destroyed because the -L option was used.
After finishing, I stopped and started the array again in normal mode, and bingo, there were all my missing files. Lost and found had only a few files from the failed MC copy yesterday. Everything else is in its place. I am now copying everything from disk9 off the unraid array.
With disk9 free, I will use it to replace the failing parity drive. And next work on replacing disk5.
Disk5's SMART status looks pretty similar if not worse than the failing Parity drive's SMART status. How can I spot the difference between currently failing and still hanging on?
Thanks again for your help and advice.