Kimmy no doubt feels he has enough bargaining power now. He can finally get that peace deal and garner legitimacy globally by normalizing his country's relations with the US. He would enter a new era and see an end to sanctions. As for nuclear disarmament, he can drag that out for decades so it comes to nothing. Logic dictates NK will never give up the only weapons it has that can completely deter western aggression.
Then again, he doesn't have much if any bargaining power. Because of the obviousness of it, there needs to be control mechanism for the US to view the deal as having any merit. They might do it as a soft strategy, hoping that the trade and economic growth would lead NK to be softened up over time. Much easier to rebel once you see signs of prosperity and future happiness, especially if it gets taken away.
More so, it's not a weapon they needed, as much as a weapon giving them independence from China, namely the Sino-North Korean Mutual Aid and Cooperation Friendship Treaty, which together with US reluctance due to the danger to a city like Seoul from artillery fire, made attacking NK not much an option.
In my opinion, the US should've responded with putting nuclear weapons back in SK again. Some analysts have warned against such a move, fearing sanctions and possible nuclear proliferation from such a move, but I'm much less convinced that we won't see more nuclear proliferation with NK's grab for attention and power.
More so, its guarantee against aggression isn't as effective as one can imagine. There are tons of weaknesses, including a covert military attack making it possible for retaliation to be hard if one can't identify the enemy country, or even more so if it's disguised as another country. If it's automated with pre-chosen targets, that allows for a third country to abuse it. Then you have new technology being developed that could invalidate their arsenal. Then you have biological weapons that could be used to wipe out NK and make retaliation harder, though that comes with a lot of dangers as well and of course a lot of dead people and disrupting the peace of the current MAD.
I'm more surprised China has allowed NK to create nukes right next door, as it's a dangerous game they're playing in length. More so if the security systems, errors or people have a chance of causing a launch. Even a large accident can, if the wind goes the right way, spread into the Korean Bay and Yellow Sea, dealing economical and environmental damage to China.
His only bargaining power would be to remove the nukes, but again, since he doesn't want to do that and since trying to drag out nuclear disarmament becomes obvious, he might stand facing far more concessions to avoid more sanctions. In a way, Obama's TPP strategy could've weakened China's economic power which could've been beneficial as means to disarm NK. Or the US might've folded out of fear of escalating conflict and aggression. Then again, Obama and Bush have dropped a lot of balls when it comes to NK leading us here. And let's not forget the Ukrainian Revolution, in which the West dropped the ball hard as well (I sat and watched and knew that Russia would interfere with Ukraine being in their geopolitical sphere of influence).
The Western diplomatic approach needs to stop in its current form, its risk averseness only seeks to reward empty promises. Weird, considering their terrible approach to Russia, which tries to sanction Russia, as if it'll achieve anything and doesn't seem like a weak attempt at exercising power. They seem to bank on rebellion amongst Putin's inner circle, but that's highly unlikely to happen. The US needs to proactive instead of reactive all the time.
