When the borders are closed and to try to leave is to be killed (and China happily sends back anyone trying to leave through their border, in order to be shot in NK). People in NK have to make the most of what they can in NK.
If someone has a modicium of Talent, then they have the choice of working for the regime or starving to death working some frozen field. So they go to work for the regime.
If you work for the regime, you get to live in a bit of comfort (compared to those starving to death in the frozen fields), but if you fail in your appointed task you will be shot. That provides a significant motivation to work really hard and find solutions.
If you're working for the regime's weapons programs, then you have the best comfort, the best funding the regime can provide, and the absolute knowledge that if you fail, you, and likely all of your family, will pay the consequences.
Based on just that level of motivation, NK dont really need China's help for any of this stuff. Maybe for some specific materials, training and hardware, but most of it they can do themselves.
If the rest of the world, wanted to really see the Kim Regime fall (so basically everyone other than China), then the best thing they could do would be to work out a way to create a relatively safe and porous border across and out of NK. The first people across it would be those that make up the scientific and engineering brains of the regime...