How do I have both custom show tv art and custom episode thumbnails?
It looks like an episode can have multiple artwork attached. I'm not sure how TV.app determines what to show for the show's thumbnail, but I've been able to Get Info for an episode, go to the Artwork tab, and select Add Artwork to add more than one image to an episode.
I haven't tested this thoroughly, but I set all the episodes to have the series thumbnail, then added an individual thumbnail to the first episode and it works as desired so far (TV shows screen shows series thumbnail and episode list shows episode thumbnail).