Aren't all bets off at this point? I mean validating the backup seems skipping steps if you are not validating the source. Scrolling through thumbnails is better than nothing, sure. But it's really prone to false negatives. Corrupted images can look good in a thumbnail and your eyes might just miss even glaring corruption because you just scrolled too fast. If it's not an image file it gets more challenging.
You seem to have one of those corner cases where basically no automated method can solve your problem but the volume of data is just low enough to alleviate the issues with a bit of manual intervention.
