John Kear is without doubt the coach you need short term, say a two years deal. He will get the basics sorted. I think JK's ability as a long term coach are probably gone now, he's had his chances but he's a bit long in the tooth now to really step outside the box. He has a weakness where younger players are concerned and is never sure of his best team when all is going well. However backs to the wall he is your man.
The fact he's a Cas lad is by the way utterly meaningless IMHO.
Whether you could coax him out of semi retirement I'm not sure but it would be tempting for him. That said JK was never keen on the Cas BOD long before all this started.
It's a tricky one but IMHO Powell is a Championship Coach however I think his local knowledge would be of far more use than any Aussie for now.
Anyway who knows this may be your low point and Mildrid will come good, it does happen.