The chemical weapons issue has been discussed to death in other threads. They are not practicing germ warfare, if they were, I'd be in a different frame of mind in this.

As to what happens if.....

If Congress votes, and turns down the resolution to use force, it SHOULD support the Constitutional division of powers and end the possibility of US intervention. However, as I'm typing this, it looks like the quisling in power in the House isn't going to even bring the measure to a floor to the vote, as Mr. Speaker knows the resolution would fail, and he doesn't want to embarrass the President. In this instance, the President SHOULD be embarrassed, as he made statements which were at odds with the strategic intentions of the United States as well as the tactical reality.

As to the G-20, the only members of the G-20 who have expressed any intention of doing anything are the French and the United States. Being in agreement with the French leadership in a situation like this should be enough to give anyone pause. Besides, Pres. Obama burned a lot of potential political capital by bowing out of Poland over pressure from Russia. Putin OWNS him, and the diplomats are well aware of it.

Syria isn't Iraq. They have been fighting Israel and their internal opponents for years. It would not be a cakewalk, especially if Russia decides to supply them with some new anti-shipping missiles in the time when Congress and the President are dithering.

As to the fractious nature of the Syrian opposition, one has to look at the awkward, forced constructs which are the current states in the area. They are artificial borders imposed by the west with no regards to historical reality or tribal areas. This is why there are Kurdish areas in Turkey and Iraq, to use one blatant example.