I'm no expert on this, but it seems the differences between ser/estar aren't exactly permanent vs. temporary, in all cases. There's some subtle differences on a case by case basis. In this particular example, I believe if you say something "está divertido" you are actually saying it's funny, whereas with "es" it means fun. I would suppose that this is because the state/nature of something either is, or it isn't, with "es." So in other words, the party could become boring for sure, but I think simply put, the nature of the party is either fun or not fun, in general cases. So I think "esta" being temporary is more for personal states of being, maybe not the general nature of a thing.
But I'm still learning – any native speakers or advanced speakers, ayudarnos por favor!
I assume i am from a very different part of England but I've never heard anyone use diverting in place of fun. Actually I've not heard it used as an adjective at all. As a verb, people are diverting traffic or diverting attention. I would say to accept 'diverting' as an adjective in place of fun could be misleading for people learning Spanish. A party could be a diversion but i don't think it could be diverting