North Korea has a very long history of breaking it's treaty commitments and violating diplomatic agreements.
That said, all-out massive attack isn't viable. there would be no way to disguise the political and military preparations for such an attack, and China wouldn't allow it anyway. Bear in mind the the South Korean capital Seoul is within medium artillery range of the border, and there are a lot of North Korean artillery pieces aimed at it. Even without a nuclear attack capability, North Korea could do them an awful lot of damage in ways that would be difficult to prevent.
The issue, as always, is cost/benefit. N. Korea has always tried to walk the line between wanting to seem strong enough that you can't just toss the leaders out and not so threatening that you actually worry they might mess you or your allies up.
It's not worth it to the U.S. or anyone else to invade, lose 30K-100K soldiers and then have to rebuild the country, and it never has been. The nuclear arms race serves partially as a deterrent against possible invasion, and partially an attempt to get people to the negotiating table for some resources by scaring Japan and N. Korea enough that they want to deal with this.
But if they had the capability, they still don't have the desire to use it as a first strike since that would give every country around + the U.S. a reason to bomb them to the ground.
This was the main idea behind the South African nukes as well. If memory serves, the country had only 6 nuclear warheads, but they were never meant to be anything but a deterrent (and probably a display of power and capability).
There are better ways to deal with this. Diplomacy ... perhaps