There is no separate preview data in an MLV file. If a preview somehow looks correct then the original raw data is intact, b/c that preview had to have come from the real original raw image data in the MLV file. I highly doubt there is image data corruption, something else is wrong, probably incorrect black level => Finder uses a hard coded back level and ignores the value in the file, so if the value in the file is wrong, preview will still show correctly.
Also, most cDNG converters (MLVFS included) don't embed a separate preview image in the DNGs (b/c Adobe Premiere and others don't like it), so the Finder is likely showing the full image data.