IIRC you don't have to use the same metadata source for everything on Plex. Couldn't you arrange your folders to make the "TV Show" in your library and swap the data source to TVDb. Sorry, no access to Plex/my collection at the moment so probably not using the correct nomenclature. I'm fairly sure that I've set a couple of things to take the data from my collection rather than an online source.

you don't have to use the same metadata source for everything on Plex

Yes, this is correct. But everytime you reset your library, you need to remind which movies/shows used which data source and you need to set all the exceptions again. The better way should be one accurate source.

I see - the only time I've reset the library was when the database got corrupted during an update, so I've never had the problem. (Ever since then I have a handful of shows and films that are always showing as "new" - even after I corrected the incorrect dates on the files! - as well as the inaccuracy of the "watched" flags.) How often do you need to reset the library and for what reason? I'm wondering if you could backup the database first and then restore it (depending on what the purpose of resetting the library was). Or just keep a list of your "exceptions".

I think you might have more joy posting your query in one of the support forums: https://www.themoviedb.org/talk , though I suspect that creating a non-existent show to keep your Plex data the way you want it might be frowned upon.