I was slightly off. It may be old news, but I haven't seen where that plot has been fully resolved, having to sacrifice members of your community to a demon created storm so everyone living in your nation doesn't die a horrible death seems like a pressing matter for 'good' guys.
As for who's where, I'm still assuming they're all a wee bit too busy to drop what they're doing. Didn't say how long it took them, but if Theya was really close enough for the mystic slayers to reach her before she moved very far with the moon guardian, I'm pretty sure calling in another, currently occupied guild would seem inefficient to deal with a threat to the world. A caste of assassins that are essentially trained to counter anything, where failure means death ( got lucky because found her), and essentially act as the faceless central intelligence agency of the council? Yep, send them to deal with the inter-dimensional magic woman.

Eh, I'm not sure that plot will be resolved ever seeing as its been 10 years and the admiral hasn't done a thing about it since the incident. Yes, it seems like a pressing matter, but if you can do something about a pressing matter you don't wait 10 years to do it.