I agree that Ukraine was not a nuclear power even while they had warheads on their territory after the USSR fell apart, but I believe it was very feasible for them to become one.

Posession is nine tenths of the law, after all-- it would've been quite possible to just lock down a few silos and refuse to hand the weapons over. Russia as a state was highly disrupted at that point, and would've had a hard time opposing this effectively.

I'm not disputing that this would've been a very costly move for an already poor nation (in potential economical sanctions and also maintenance of the arsenal itself). Maybe the external political/economical pressure resulting from this would've ripped Ukraine apart some other way.

But I'm highly confident that Russia would not have risked annexing territory from an country with a few nuclear ICBM silos. No need even to have full control/launch capability, as long as there is sufficient doubt (on Russias side).