1) It's complex. Formally, Moscow controlled the launch codes. However Ukraine designed and built the ICBMs, and are near the top of nations with the highest nuclear physicist per capita ratio.
On top of that the Soviet nuclear lockout systems are rumored to be much simpler than the American ones. Whereas the American system is rumored to be something like the decryption key for the detonation timings (without which you have at best a dirty bomb), the Soviet lockout mechanism is rumored to just be a lockout device with a 'is locked' signal going to the physics package. If that's all true, taking control of those nukes from a technical perspective would be on the order of hotwiring a 1950s automobile.
Taking physical control would have been more complex, but everything was both more complex and in some ways a lot simpler as the wall fell. It would have ultimately been a negotiation.
2) See above.
3) Which military nuclear power has been attacked by the kind of adversary that you can throw a nuke at? Yes, it doesn't remove all threats, but no solution does. Removing a class of threat (and arguably the most powerful class of threat in concrete terms) is extremely valuable.
