That doesn't negate the possibility of Assad (or his subordinates, clandestine organization, mole people) from having used those weapons, nor justify it depending on the point you're making. Same goes for Assad (or the mole people).
Nobody knows a damn thing. The US and Russia both claim they have evidence damning the opposition of their interests, while not disclosing said evidence and running around inquiries. There's no clear picture on where these stockpiles are if the facilities are truly out of commission and no longer a factor, so there's lack of a specific target. Targeting city centers would be disastrous, as the civilian toll would have the US facing heavy sanctions that the UN will have little to no choice but to enforce. And let's not ignore the elephants in the room that are China, Iran, and Russia, of the last who is making it clear that they have the capability to and will likely respond.
Arguing for one side or the other is the behavior of fools given that the outcome of either side coming out on top will not be pretty for anyone. Assad wins, terrorist attacks will likely continue in the region. Rebels win, and it's likely we'll see a repeat of when the Mujahadeen forced out the Russians in the sand box.
I don't think we can call the current FSA the same as the initial FSA. They weren't spring buds in the beginning either, but that's civil war for you. I'm friends with quite a few Bosnians who fought in their civil war a couple of decades back, and it's not as black and white as many like to think. One moment you're fighting for one side because that's who holds your city, and the next day it's the other. Alliances change, and you can't turn away the devil when you're getting your ass handed to you. You're fighting to keep it away from your city, or your neighborhood, or your house even. You do what's best for your family, and many times you're forced into that path through threats or honor/shame. You don't want to be the guy who says no when recruiting comes through the block.
This is probably the best reason for why we should stay out of it, but chemical weapons are a serious issue to the UN and are illegal. Standing by and doing nothing about that particular issue when sanctions will have no effect undermines what many already consider a useless global authority, for lack of a better term.