What do you mean? Russia does not rely on oil and gas imports from Iran, it is a major net exporter. What Russia does import I'm pretty sure mostly comes from the spot market and I think the situation is greatly improved from where it was a few years ago when they were pulling in something like 100,000bbl a day (I think it's down to the 40,000 mark). Nor is Iran the major conduit for Russian exports so I'm not sure what you are referring to. I'm also not convinced that Iraq figures prominently in Russia's strategic vision, it is fairly far from their potential orbit.
Why is Russia concerned about Syria? For fairly simple reasons. The regime in Damascus and their tiny little naval station at Tartus represents their last real ally and toehold in the wider region. Since 1990 they have seen the collapse of allies (strong or erstwhile) in Yemen, Iraq, and Libya all the while the GCC has strengthened which has further reduced Russian influence in the immediate region, the rise of Turkey has curtailed Russian reach across the northern Middle East and challenged them in the Caucuses, and on and on. The American and Western position has appeared to grow stronger and stronger and with the fall of Assad it will remove Russia from the region entirely except as a peripheral power and as an interlocutor with Iran. Of course there are ancillary reasons some of which you mentioned (bolstering secessionist or Islamist movements in Russia, underlining the doctrine of humanitarian intervention in foreign policy, historical animosities and narratives, etc) but the aforementioned one is the primary root of Russian fear.
The global board has been turning more and more to the liking of the West and Russian efforts have thus far completely failed to halt this trajectory. Losing Syria would be another piece knocked to the ground.
