I think you argument is flawed ... It's not like the most powerful nation on Earth needed "incontrovertible evidence" to invade Iraq. And why would the secular government of Syria take such a risk?
Invading Iraq is an inconsequential minor operation in comparison to incinerating Syria with the resultant radioactive cloud drifting across more than half of the Mideast.
I seriously doubt any nation, with the exception of NK, that currently has nukes would use one without incontrovertible evidence that garnered the support of most of the other nuclear powers. One could expect China and Russia to oppose the use of nukes even if 5 major US cities were smokin' holes.
Syria, not a secular government as you posit, might take such a risk if they convinced themselves they could hide their fingerprints well enough to avoid any sort of proof.
I'm sure Evenhaim will oblige you if you wish to argue that Syria's government is truly secular.