You are wrong. The worst a terrorist is ever going to be able to do is a dirty bomb - basically a bunch of C4 next to the radioactive material. The bomb will spread radiation across one or two city blocks and that's about it.
The reason that they will never actually detonate a real nuke is that they are complicated and extremely delicate. The shape of the bomb must be absolutely perfect and the timing of the charge detonations must be accurate to within microseconds, else nothing happens. Getting the shape right is so important that people working on at least one major nuclear programat Los Alamos had to classify all spheres, including oranges.
It will take the resources of a nation-state to blow up a nuke on US soil and no matter what any war-mongering politicians have said, no actual nation-state is stupid enough to do that because it means the end of that country. Not Iran, not North Korea. Not going to happen.
I don't know about that. Most of the problems in shaping it comes down to having the machines to craft and shape the bomb to tight tolerances. We've been able to keep the machines that can make objects and refine materials with such tolerance out of foreign states for the most part. That's what has saved us as much as anything. The math is pretty much out there in the open to a degree. With 3d printing and 3d shaping (lathes/cnc/etc) I don't think we are far from being able to shape any material into any shape. And there are explosives that could certainly be printed and shaped to the nth degree. With some of the new technologies going mainstream and available from all quarters it's going to be downright impossible for us to control those technologies. I think the chances are going to be much higher.
Where there's a will, there's a relative.