it is in our national interest to deter the use of chemical weapons in the region.
Can you outline how this could possibly be the case?
Is it in our national interest to encourage the use of chemical weapons anywhere? I cannot imagine that ever being true.
You are failing to demonstrate your case. There is a third possibility - it neither in nor against our national interests if chemical weapons are used in the middle east.
I really think cost/benefit analysis in this situation is firmly on "do nothing" side. Lets just assume that was Assad's weapons used intentionally. How likely is it that he would conclude civil war, turn around and start attacking US with these chemical weapons? Astronomically unlikely, we have ocean in between and we have nukes. Even if he wanted to do a suicide run, at best he would kill only couple thousands.
Now, what are costs of intervention? We could misread situation due to fog of war. We could accidentally cause regime change, and that will likely lead to genocide. We have seen this unfold with Yazidi, no reason to expect this will turn out differently.
An analogy - Hitler and Vlad the Impaler are waging civil war. What do you do? You know Hitler will start genocide, while Vlad the Impaler will only impale people that challenge his rule. So your real choices are - side with the lesser evil or do nothing.