[isarl]

This topic is not about developing Calibre, it is about intentionally trying to work around a core Calibre design decision that is well documented, explained, and not changing. So really your request seems to be about how to use Calibre, not how to develop it.

Calibre has very good reasons for being opinionated about the way it stores the files in its database. If filenames and directory layout is important to you, you can get the GetFileName plugin, which stores the original filename as metadata when you add a new book to Calibre. Note that the imported copy stored in your library will still be renamed by Calibre and you should stop trying to change that.

You can also use Calibre's export functionality in combination with the custom column you create to store the original filename. If you use this column in the save template in Preferences > Import/export > Saving books to disk then you can recover the original filename when you export from Calibre. Note that this is for exporting your catalogue – the files in Calibre's library will stay named how Calibre wants.

Again, however, it is inappropriate to try to control how Calibre choses to store them inside its own library. Let Calibre manage the files in its database, and adjust how you export catalogues from your Calibre database.

Good luck!