I don't understand what your argument is. I never claimed that it was impossible to develop nuclear weapons if you don't already know how to do it. That every country that has attempted it has succeeded is not the same as "there's a recipe book you can find online that you can just follow to the letter and build your own nuclear bomb, provided you have the resources". If such a book existed it would drastically lower the barrier to build a nuclear bomb, because you could skip the science part and just follow the recipe, certain that it would work. To be clear, such books exist for drug manufacture; they exist neither for semiconductor manufacture nor for WMD manufacture.
The hard part has seems to be the metallurgical process of enriching the material (and doing it in secret), not the actual building of the bomb. I bet if you asked any physics grad student they could build you a viable bomb.
What do you mean exactly? They could build something that goes boom, they could build first try a 100% yield fission bomb...? Just because someone builds an explosive device that incorporates fissile material into the design doesn't mean they've cracked the problem. I bet I could build a "viable bomb" if you give me the resources, I just can't say with any certainty it won't fizzle or it won't be a dirty bomb. Can you do your deterrence with a warhead filled with C4 strapped to uranium ore, while I use the money saved to go on vacation?
I mean the trinity test didn't fizzle out. Seems like most bomb tests went off without a hitch first go. Again these were mostly teams of physicists under 30 years old doing this work. I would guess "how I would build my nuclear bomb" is a pretty ever present thought experiment for nuclear physics grads. And if you were empowered by a state to solve this problem with all the resources states typically devote to their own nuclear programs, it just won't be a matter of "if." Once again, no one who walked down this path has failed really. The secret sauce is probably boringly simple and readily apparent in small scale experimentation.