Opinion here amongst the political elites seems divided on the basis of “legality” being that there was no UN mandate for such action, however, as it’s been pointed out to the opposition party, since as it was probable the Russians would veto any such proposal the nicety of having a mandate was unlikely.
There will always be the back and forth here about the legality. Congress is split on the issue, every President since its inception has declared the War Powers Resolution is unconstitutional, the Supreme Court has shown no appetite to take the issue up. Unless Congress acts to clearly change the dynamic then for all intents and purposes the President clearly has the power to do what he did.
I’m rather on the fence with these sort of “slap on the knuckles” actions. If one is opposed then do something meaningful. What I mean by that, well, even I don’t know as it’s difficult to see the picture that’s emerging, however, I’m convinced that such hit and run tactics whilst they may serve a political narrative achieve very little. To be brutally honest I’m net even sure I care!
I think you are correct. Had chemical weapons not been used (over and over again) I don't think anyone would fundamentally care about what is going on Syria. On some level this is really a debate between the West and Russia and its not really about Syria at all. It's Cold War politics all over again.