I'm currently working on implementing a verification system and this is what I'm currently thinking.
I'll add a 4th crop method called verify posters or something like that which will check if a poster file exist, check if the dimension matches(with an option to add a margin of error) the expected ones from the software cropping and it'll take into account the crop threshold too as a sanity check for when it crops too much, which means it has a high chance of being wrong. It will then only display those that break those checks which means there's a good chance of something being wrong with either the existing poster or the software crop.
It will also now always load the poster file(which means it's possible to check them all one by one even outside of the verification mode) if it exist so it can be compared with what the cropping software does with the current settings which looks something like this right now:
If there's any ideas or concerns about the way I'm thinking of doing this, let me know.
I've debated doing a more manual verification on more than one poster at a time but decided against it since ideally you need the original cover to compare since some are less obviously wrong than others and they'd need to be fairly small to find a decent amount on a monitor.
I've also improved the edge detection a bit by setting a minimum % of the width of the image allowed(only for the automated detection, manually changing the size has no limitation) for the poster, which eliminates the single digit pixel wide posters. It tried to detect only on the right side of the image if that happens, which helps detect better. The cover in the example above had a 1x1 pixel poster made before.