Pagan gods shouldn't be in Purgatory either, should they? They are not children of Eve. But the writers probably don't care too much about logic. If they want to bring angels back, and Purgatory feels like a good way to do so, they'll just twist the mythology. After all, it was never actually stated that angels DON'T go to Purgatory.

I wonder though, do leviathans go back to purgatory after they die? So we'll see Dick again?
Apparently. At least that's what Cas hints when he tries to explain why they are in Purgatory: "And where would he [Dick] go in death?"

I'm not sure it was too difficult for Cas to kill the pig because its death wasn't pointless?

Cas was amazing in this episode, and I really think he's getting better and may actually be okay again, eventually. But yeah, Dean is still acting like an ungrateful douche and I am so tired of it. Lucky him that Cas loves him enough to stay by his side anyway.