It's not clear to me why Iran is a part of this multipolar order. It's true that the U.S. has backed Saudi Arabia/GCC and more or less taken the Sunni side in the Sunni-Shi'a split, but why is Iran considered an independent world power in the multipolar world but Saudi is not? In the same way, Cuba and Yugoslavia were not powers in a multipolar world in the 1960s just because they stood against a U.S. The hegemony but were nevertheless not explicitly owned by the Soviet Union.
The U.S. definitely has an interest in an isolated and weak Russia, but the polarity of this situation is totally unclear. Isolating Russia has already empowered China. It may strengthen the EU and EU-central/eastern European ties, but it may also cause a further loss of influence in Africa and SE Asia for the west. It's definitely possible to make the math shake out for this to be strategically useful for the U.S., but I don't think it's as foregone a conclusion as you expect it is, and for that reason I don't see how this is any more likely an explanation than any contradicting explanation.
The U.S. definitely has an interest in an isolated and weak Russia, but the polarity of this situation is totally unclear. Isolating Russia has already empowered China. It may strengthen the EU and EU-central/eastern European ties, but it may also cause a further loss of influence in Africa and SE Asia for the west. It's definitely possible to make the math shake out for this to be strategically useful for the U.S., but I don't think it's as foregone a conclusion as you expect it is, and for that reason I don't see how this is any more likely an explanation than any contradicting explanation.