BS, if I may make a suggestion:
Much as they would like to keep their foothold in the Med, the Russians can probably see the writing on the wall. The evidence regarding chemical weapons and who used them is becoming pretty overwhelming. In the circs, they can hardly keep saying 'nyet' and remain credible, let alone contemplate taking any military action in Assad's favour.
Don't measure Russian wheat with your bushel.
Per the bolded part: credible with whom?
Whose opinion does Vlad Putin give a flying fart about?
A thought:
The Russians and the U.S. have a major shared interest, which is combatting the Islamist/Salaafist movement that has caused both our nations no small amount of trouble. That this common ground cannot be a place to begin agreements on a shared policy with that in minds is a disappointment, but it is not fatal. In the longer term, we may be able to use that common interest as a way to get past a few of the latest dick measuring escapades that have passed for negotiations and diplomacy.
Back to Syria:
The Russians have modest skin in this game, if any, but also have very little risk in the game. They don't need to do much to help Assad remain in power, regardless of what weapons he has or has not used.
But, if it is convenient for them to do so, they can drop him like a hot potato with minimal cost or risk to their position.