No, a new START treaty is not key to addressing the Iran problem, because credibility is more important. The United States and other nations needs to clearly send a message to Iran that they are not going to tolerate a nuclear Iran. It does not matter if there is a treaty or not if the United States is credible with this message.
The Iranians should be free to do as they please in regard to nuclear arms. All nations with nuclear aspirations pose no greater threat to world security than those with nuclear arms. Treaties on nuclear arms reduction have nothing to do with nations without nuclear arms. Any attempt at thwarting nuclear proliferation is an attempt to control those without nuclear weapons for the advantage of those with nuclear weapons.