Good Points.
I had totally overlooked the Islamic angle, although it is a pretty obvious one. Russia doesn't need Iranian oil, but I still bet it would like to deny it to us, just on general principles. It also wouldn't hurt to have allies in that part of the world, especially rich ones with influence and power. But I think this time the Russians may have outsmarted themselves. Iran is a loose cannon. At one time it was our client state,
and things changed. Russia should be wary of an unstable and powerful Islamic nation on her southern flank, for the very reasons you mentioned.