Why do Russia and China veto the resolutions concerning Syria?
Why does anyone veto anything? Because they benefit from the status quo more than the alternative. Russia and China consistently make a point of using their UN Security Council vetos against the West's ideas, in part because their politicians benefit domestically from taking a stand. In Russia and China, being seen as a puppet to America's goals equals political suicide. Much more than that, though, Russia and China have large economic stakes in the current regime, and a bloody revolution could cut all that out from under them. Russia, for example, would never get paid back the billions the Assad government owes them for weapons. As allies to Syria under Assad, quite divisible, Russia and China have to show their support for the government by making Western interference in the form of resolutions and sanctions as minimal as possible. Additionally, due to the outcome in Libya, China and Russia are now taking a more non-interventionist attitude than ever. Though the UN has a declared "responsibility to protect" people from abusive governments, as The Economist says, "China and particularly Russia, feel that the 'responsibility to protect' has already gone far enough, thank you very much. Last year, they signed on to a resolution that authorised 'all necessary means' to protect Libyan civilians from Muammar Qaddafi. That intervention became a NATO-led air war against Libya's regime, and ended with Qaddafi's bloody death at the hands of the rebels. The Russians felt duped."
Could there be a potential backlash from Iran if we (I say "we" meaning the UN) do end up taking the path of military intervention?
Syria is now tenuously operating under a UN-monitored ceasefire, with the Assad regime not making the peace concessions they promised. However, despite the continued killing, I don't think we will intervene militarily unless something drastically changes. The UN will never get the vote needed to intervene–with the Libya situation, the vote passed because Russia and China abstained. As I said previously, Russia was not pleased with the way the Libya campaign was run, and any military venture with Syria would be exponentially more difficult as air-strikes wouldn't be effective on the large scale that they were in Libya. That leaves NATO and the Arab League to take the lead, and NATO already declared that they had no intention whatsoever of doing so, and there is no way that the Arab League would independently take an action with such a high likelihood of sparking regional chaos. Even less likely is the idea of the US taking a unilateral action of military intervention in Syria, especially with Obama up for reelection, and the economy as the forefront issue–this would be a bad time to insert ourselves into a(nother) long-term war. An alternative idea being tossed around is arming the Syrian opposition, but caution is warranted, because as The Economist puts it, "The guns that flooded into Afghanistan to arm locals against the Soviet Union helped create the chaos that spawned the Taliban," and this is a very similar situation.
Essentially, what I'm saying is that as things currently stand, I don't see a military intervention happening in the first place. But in the hypothetical situation that one did, there would be tremendous backlash from Iran, as they are longstanding and very close allies, who have recently been seen working together militarily.
Aside from Russia, China, and Iran, which other countries are known allies to Syria?
Currently, no one other than Russia, China, and Iran. Syria has angered the European Union, America, and the rest of the West, and gotten themselves booted out of the Arab League as a consequence of using deadly force on protestors after declared ceasefires. If you're interested in who has a stake in Syria's fate, I recommend you follow the money, and for who supports the regime, follow the arms trade.
What is the estimated number of deaths in Syria now, and how does it compare to Libya at the time when we decided to involve ourselves?
Somewhere between 9,113 and 13,368 dead in Syria compared to (?<10) thousand dead in Libya previous to Coalition intervention; the Libya number is hard to ascertain due to media lockdowns and still-uncertain missing/dead counts. Remember, though, the decision to intervene is not something based purely in casualty statistics; rather, it depends on the history of the region, the political climates of each nation involved, potential consequences, alliances, promises, treaties, and a host of other factors.