I think you are underestimating North Korean regime.
It is a bit old fashioned politics but Kim Jong Un is playing by the books when it comes to the old Asian political advise.
The whole country relies on the control of the people. If people thinks the country can be toppled, it will be. The government has pretty tight control on the communication with outside world, so they have that covered (although I hear that South Korea Drama, music, etc. circulates in the black market in DPRK these days).
If they don't know what is on the outside, the next thing to make sure is to not make the government be seen as weak. This is especially true at times like these with rather recent power change - From Kim Jong Il to his son.
It is a smart move for Jong Un to execute any possible dissenters or somebody who could gather enough support to overthrow his rule, potentially his uncle. Even if the uncle has no desire to overthrow the government, it is probably a good idea to make him a scapegoat and discourage any other who may think of such plan (ruthless, I know, but such thing is common in Chinese/Korean royal history).
As for some Americans being sent to work camp, it is to show to its people (and outside missionaries) that communication with outside will be strictly forbidden. DPRK is showing (not telling) that if you get caught, there will be severe consequences. There is no doubt secret Christian missionaries to North Korea has taken a step back for now until strong anti-missionary policy calms down a bit.
As for missile tests, hostile interaction with neighbors, it is probably used as a propaganda to its own people, occasional reminder of the military strength - to control the people, either through fear or through nationalistic pride.
Kim Jong Un is not a person who has been stuck in his country with no clue how world operates. I do not think he or his staffs are stupid people. They do things to accomplish their goal: maintain power. I believe all their stunts are calculated moves to keep his people in line. Sometimes they piss off neighbors and U.S., but they never do too much to actually escalate things to war. They can get away with that because well, they don't care if Japan publicly denounce them, at all.