By John Irish and Francois Murphy VIENNA (Reuters) – Parties trying to revive the Iran nuclear deal scrambled on Wednesday to resolve last-minute Russian demands that threaten to scupper negotiations, diplomats said, with the United States appearing unwilling to engage with Russia on the matter. Western powers on Tuesday warned Russia against wrecking an almost completed deal on bringing the United States and Iran back into compliance with the 2015 accord. Iran’s top negotiator returned to Vienna on Wednesday from consultations in Tehran. Russia’s envoy to the talks, Mikhail Ulyanov, dismissed…