Are just reasons for countries to not want nuclear weapons. But if e.g. Japan announced a nuclear weapon program (or it was uncovered by foreign intelligence) I highly doubt that anyone could/would stop them-- hinder their economy or facilities with sanctions or sabotage, sure, but outright stop them from becoming a nuclear power? Unlikely.

IMO every bigger western-ish nation state has the ressources required to build nuclear weapons; they just lack motivation to do so.

Just consider that smaller states like France were already able to do this in the 1950ies, and relevant helper technologies (computer aided design, simulation, industrial control) are MUCH more accessible and advanced today.

Fusion research, on the other hand, is not going to help with the major challenge in becoming a nuclear power: it does not help with obtaining highly enriched fissile material.

If countries wanted to throw research funds at becoming a nuclear power, it seems infinitely more likely to me that those would go into "innovative reactor research" with the endgoal of producing HEU or plutonium.