With proper naming, Plex should pull the right metadata and posters for you.
That being said… if the media used to be matched wrongly in the first place, you might want to perform a “Plex Dance” to ensure no old cached information gets stuck.
If Plex keeps not giving you the posters or misses out on the metadata from some episodes, this could mean there’s something stuck in the cache of the TVDB agent.
https://support.plex.tv/articles/202967376-clearing-plugin-channel-agent-http-caches/
As for media deletion… if you allow Plex to delete items directly from within Plex, this will actually delete the media (that’s basically the only aspect where Plex actually messes with your files).
Check out Settings > Server > Library > Allow Media Deletion (“The owner of the server will be allowed to delete media files from disk”).
Personally I recommend to uncheck this option.
The way to stay in control over what media to be included in Plex is by moving it in or out of the folder that is linked to your Plex library. After you moved a file outside of the folder monitored by a Plex library, you click on Scan Library Files for that library and the records in Plex will go away.
Side note: If you have disabled Settings > Server > Library > Empty trash automatically after every scan, the record might still show but have a trash can icon over the poster (indicating the underlying file is gone – this is useful if your files are on an external drive and you don’t want to accidentally lose your Plex records by doing a library update while the drive is not connected / takes too long to spin up).
https://support.plex.tv/articles/202606363-how-do-i-delete-something-from-my-library/
https://support.plex.tv/articles/200289326-emptying-library-trash/
