I think the North Koreans had lots of information going in to avoid pitfalls and problems. They would want to deploy small warheads that could be used in efficient ways.
Rocketry is a different type of science/art.
The North Koreans might have the ability to build a large, crude atom bomb. They will never be able to build one light enough to mount on a long range missile. A thermonuclear weapon is farther off than their future.
I doubt it. If anything, they would have a lot of rocketry info too, and from the same sources - and have we seen any of that? How long did it take them to progress from the old Scud to their most recent fizzle? Decades.