I'd rather not get into my opinion as to why we should intervene more broadly in Syria because I think it would be distracting but there are a plethora of other threads where I've elaborated on that I could link you to.
1. I completely disagree. The President's credibility is the credibility of the United States. The question being asked across the region and in foreign halls of power is whether or not the US has the stomach to flex it's muscles after Iraq or whether we are fatigued. Even if we do little it is important that we show that we act.
2. We would be demonstrating that the use of these weapons warrants special punishment. Moreover we do have the support of Western nations, military support from Turkey and France, diplomatic support from the UK, Canada, and Australia, and potentially further military support if we chose to act. The issue is not our displeasure, it is that their usage incurs punishment that they can ill-afford to have repeated.