Either disable all notifications in Mail and set it to never pick up mail. I can’t give more details - I don’t even know if it is possible - as I don’t use the app. Or disable / uninstall Mail
Sorry I have no idea. I haven’t used POP3 for a very long time: all my email accounts support IMAP
If you delete / uninstall it (as described in this post) then you don’t have to bother turning off notifications and sync. You won’t reclaim any usable storage however, as it is installed in the system partition