[ There's a reason why Shirou was invited. If the outlanders didn't have personal feelings about Teyvat's struggle, then it would be difficult to motivate them to do anything. ]

Yes. Abyss has never stopped trying to cross over into the material realm.

[ Especially since they have had several leaders since that day five hundred years ago. ]

Once your body becomes accustomed to Teyvat, I'm sure your smelling ability will return. The magic here is so much more potent and concentrated than other worlds', it tends to drown out foreign abilities. But you'll regain them over time, as long as they don't depend on you being in your world to function.