@brion - Any ideas on this or who would be good to ask?

Sounds like some of that sequence resulted in a conflict that didn't get handled well at the time. Following the error message back to the code symptom, the case is this in LocalFileRestoreBatch.php:

			if ( $row->fa_storage_key == '' ) {
				// Revision was missing pre-deletion
				$status->error( 'undelete-bad-store-key', $wgLang->timeanddate( $row->fa_timestamp ) );
				$status->failCount++;
				continue;
			}

It's labeled as being caused by the file being missing at deletion time, but specifically means there's an empty slot for the file's deleted filename (SHA-1 plus file extension) in fa_storage_key, so it can't be found in storage... In principle if the fa_sha1 entry is present you might be able to reconstruct the key though, if the file got into the deletion archive somehow (though a conflicting deletion perhaps).

Looks like both the fa_storage_key and fa_sha1 are blank, so no luck :(

What does Commons have anything to do with this task?

@Masumrezarock100: You have to ask Steinsplitter in T118234 about that, as it was added by a Herald rule.