Just thinking out loud on this. The Russians have a big interest in keeping Iran within their sphere of influence. They have little or no interest in "defending" Europe. So you're proposing that they be induced to lessen their influence in Iran by "allowing" them to "defend" Europe, thereby eliminating the threat of American missiles on their borders.
Isn't that like asking them to capitulate to American blackmail? "We'll undertake not to put missiles in Poland and (is it Slovakia? or the Czech Republic?) but you have to piss off the Iranians for us."
Why not answer this way: "We'll do what suits our best interests in Iran, but if you persist in installing missiles on our borders, we'll (a) put the screws to your allies Ukraine and others by restricting their energy supply and/or (b) target Warsaw and (Prague? Bratislava?) with nukes just in case. We might just have to begin a new arms race to overcome your new missile defences."
I start from the proposition that the new missile shield is disguised aggression against Russia, whereas Russia's courting of Iran is just basic defensive manoeuvering aimed at preventing aggressive moves by Britain and the U.S.A. , for which there are historic precedents. Russia is not the aggressor here. Any "compromise" they may be moved to make would be a victory for American aggression, veiled though it may be. Putin doesn't look like the kind of guy who is easily intimidated or pushed around. I don't think he'll fall for it.
