If I understand correct the present situation is that cover art is cached and Media Player first looks whether there is cached info and then displays that. I think if there is a (minor) change then several problems will be solved.
I want to suggest that MP first looks whether there is a movie filename.JPG within the folder and if that is available then display that, then check whether there is cached info then display that and if no cached info is available then checking the internet and caching and displaying that.
In that way it is easy to replace the cached info with user-specific and there is no need to either erasing all cached data or tedious middle button to override cached data.
When first caching cover art/metadata for your files MP will look for user supplied cover art and metadata. If available, if will cache that for future use.
If no user files are available it will try and download this info from the internet.
If no info can be found online, it will generate a thumbnail image from the video file itself.
Once the info has been cached, user supplied or otherwise, you will need to reload metadata for that file to update both the cover art and/or metadata. (Note: The option to reload metadata can be accessed by holding select when the file is highlighted).
when you say that the media player generates a thumbnail from the video file itself, is there a setting to tell it how deep in the video to take the snapshot? like take the snapshot at 8 seconds, or 25 seconds?
why is it that some folders dont generate covert art?