My point is, the USA could very well obliterate NK nuclear facilities (possibly using nuclear missiles), if other countries in the region approve. Even the 5th land army can't defend anything against missiles. I don't believe it is that easy to hide secret bases. How do you hide trucks etc ? Also images can be analysed automatically, which means looking at every square inch is actually possible.
Besides, modern conventional war is all about air anyway. Note that I'm not talking about guerilla here.
Finally, NK doesn't live in complete autarcy. They rely on external help for food mostly, and still export missiles (that is arguably the only export industry in NK).
Something interesting to think about is what would the international community do if a disaster similar to the Haiti earthquake happened to NK right now.
The conditionality you provide for the USA being able to obliterate nuclear facilities is the main problem precisely, very few countries would approve of using force within another country's border. Sovereignty is still unconditional for a lot of countries and especially for countries like Russia and China. Not the USA or any other country is willing to take the bad rep for doing such a thing. And that is just talking about regular missiles, nuclear missiles are not even a possibility.
If you put a nuclear base in a mining facility, who is going to see what goes on in there? Who is going to see the difference between trucks carrying iron ore and enriched uranium? Then there is also the fact that any country is allowed to use and produce nuclear energy and that any country is allowed to produce missiles. You can't bomb a country for doing any of those things and the link between the two is hard to establish. Case and point Iraq and Pakistan.
It think it is well possible to hide such a facility and it is also possible to make such a facility impervious to air assaults even with nuclear missiles. To protect from radiation you only need 60 cm of concrete. If a country is able to produce nuclear bombs, they are surely able to protect them from air assaults. You do in fact need a ground assault or some covert ops operation to take them down. Furthermore facilities will be spread out and there is not any guarantee that the destruction of the facilities will do anything more than slowing them down a little.
I didn't state that North Korea lives in complete autarky, they are willing to though. They don't let any amount of economic sanction stop them in their military pursuits. They do not live in complete autarky because the international community does not want the Nortk-Koreans to starve, but sanctioning is still severe.