As you've already stated, that Sarin was used has been confirmed by autopsy reports etc. Mushroom clouds are made up of debris and smoke etc, caused by the impact so it would hardly be colourless.
Who paid for the autopsies?
Why a yellow mushroom when most building materials aren't yellow?
Why is the Russian claim about rebel held sarin invalid when Carla Del Ponte says it isn't?
Searching for truth isn't denying any given set of events happened, it's just searching for truth.