Your argument rests on the assertion that possession of nuclear weapons would increase the hold of the Iranian regime on power within it's own country, and yet you fail to explain how could that be.
Nuclear weapons are hardly crowd control devices: you don't put down a rebelion by nuking the shit out of them. Even the most tightly controlled nuclear explosion would affect the whole country due to radioactive fallout.
In fact the only path I can see that would keep the current regime in power for longer due to them getting nuclear weapons is if the aquisition of nuclear weapons increased the pride of their people in the Technological and Military achievements of their country, hence supporting the regime. However, that being the case, the regime's hold on power would effectivelly be coming from the will of the majority of the Iranian people rather than from oppression.