This is a fair question and all of your critiques here also strike me as valid and serious ones. I don't have a great answer. America has not been very successful in promoting democracy. Hell, it took us like forty years just to figure out that you can't shoot people until they become democratic.
At the same time, though, I think there's a strong argument that many aspects of the post-WWII global order depend on the assumption that America, and the UN, will act to stop genocide, oppression and conquest. Admittedly this assumption has not consistently held true, but it hasn't always failed. We know what the world looked like before WWII and it mostly sucked for everybody. Hard to know to what degree Pax Americana is keeping that from happening again.
I think the question is whether you think it is possible to ever have an effective and appropriate military intervention, and if so, what would be required. Your position can certainly be "no, never," but I don't think I agree with that. It's too easy to postulate situations where we probably should act. The problem is that real life is rarely like postulates.
Obviously in this particular case, at this moment, I don't think the US should militarily intervene in Syria, for the reasons I've given. Staging a fake reality TV intervention in Syria is, if anything, worse.