#2: There's no such thing as "Traditional Korean" (just South Korean, and North Korean), so saying that they don't speak "Traditional Korean" in NK is pretty severely misunderstanding the language. The Korean locale/encoding is very easily changed as a setting, obviously, but it's also possibly used by NK hackers because it would let North Koreans type in Korean. Saying "North Koreans don't speak the same kind of Korean as spoke in South Korea, and thus wouldn't have a Korean locale set" is kind of like saying "Americans have their own dialect of English as distinct from the British, so they probably will not have any English-language keyboards."
#4. Doesn't the fact that the hack seems to be for retribution and not for personal gain precisely fit the motives of a propaganda-oriented country? Would you expect a dictatorship that works on propaganda and political influence to really negotiate some sort of deal with Sony?
My bet is that it's some mercenary hacking group hired by NK, not necessary a state-sponsored thing but by a few high-up individuals within NK --- but not by North Korean hackers themselves.