Semaphore.
Sondrestrom is listed as an ETOPS diversion field. But the minima are very high....2635ft and 3600m. I haven't seen it used as a suitable airfield on any flight plan I have used as there are invariably better choices. I imagine the facilities for 300+ passengers are fairly limited also.
Don't know about question 2, but for question 3, see Frank M's reply below.