It’s not possible to repair missing files and rebuilding will just skip them. However, repairing should add the orphaned files again to the database. You could select the failed files in the Log window, choose “Show In Finder” in the contextual menu, move them to the desktop and try to import them.
You can open the DevonThink log (journal) and find the complete path and name of the orphaned files. Then you can “Show Package Contents” within your database file, and retrieve and move the 18 orphaned files onto the Desktop, whereby you can reimport them into DevonThink.
As for the missing files, I am not able to help as this has never occurred on my system.