Please post one request per thread, that way we can keep track of the requests more easily.
The file dates are the date the pics were added to the coppermine database, it doesn't read the dates from the filesystem. You can modify the ctime field in the database, but I can't guarantee that will work for all cases.
I expect Coppermine could be modded to look to a specific folder for thumbnails, but you could have files named video19.mpg being different videos in different folders and there would be no way to specify a unique thumbnail for each one using that method.
I don't know what you mean by 'gallery link photo'. Most thumbs that you can't assign manually are the last added pic.