Once you've solved all those physics/science/engineering problems you have to then solve a whole new set to make it small enough and rugged enough to transport to your target. Something between a shipping container and a small cone on the end of a missile - depending on how you intend to deliver it.
All that is a lot of sophisticated science and engineering for a Country that has demonstrated it has a hard time with (relatively) simple "rocket science." Though they are getting better, in fits and starts, at that. They are probably advancing in nuclear weapons technology too. Hard to gauge progress (both for us and them) when they aren't testing actual devices...
The Iranians supposedly obtained blueprints for a compact warhead when a former Soviet nuclear scientist deviated from the original plan. They already have ballistic missiles. Now, the only missing piece is the material. It will be a disastrous if they figure out a way to assist the Norks with the plutonium separation.
No doubt any cooperation would take place in NK. Iran could open their domestic facilities to inspection and claim they're abiding by the agreement they made with the current admin.