Hmm...
I'd rather first address the "lame duck" point. Why is it such a bad thing to pull back from military action when an alternative seems to have presented itself? Even as a supporter of intervention in principle I'd still see it as a last resort. The plan to withdraw chemical weapons from Syria, if carried out properly, would certainly be useful and would then allow us to focus on the vital humanitarian aid that is needed. I suppose that it could be a distraction attempt by Russia to keep the US out, but then most people were in favour of (at least the UK, and I assume by extension the rest of the Western world) keeping out so isn't it a good thing then that an opportunity has arisen not to go to War? Obama has, in my opinion, done the right thing by exploring it, although I hope that military intervention remains on the table should this plan fall through. Rather than being a "lame duck", it sometimes takes more strength to change one's mind than to stick to one's guns.
As to Putin's claims -- well, I think he's being far too definite about it but otherwise it's hard to disagree. A strike on Syria could result in more victims, and could spread the conflict wider -- by becoming a proxy war against Iran, for example, or even against Russia given that both of these countries support the Assad regime. On the other hand Putin seems to have given little attention to what not striking could do. The situation is already awful for those involved, and talking and negotiations have long since stopped being options, as long as there is no pressure from outside. Too, what does not intervening say to (some of) the people of Syria as well as to those despotic leaders who might be tempted to follow Assad's lead? To the first, it says that the West is no longer willing to come to their aid when they need it most. Ironically, this could push people towards Islamic extremism just as much as intervening to allow such groups to win might -- because some people, with no support from the West, might see only the support they gained from those extremists. Intervention could do the same thing too, of course, in a different way, because others might perceive any strike as an attack on Islam. It's Catch-22, really.
To other despotic leaders, meanwhile, no intervention would say that they can carry out such grotesque and barbaric attacks against their own people with impunity. This doesn't seem helpful either.
Where I disagree with Putin most is -- well, firstly his claims about "We are not protecting the Syrian government, but international law," are a bit of a joke, really -- the certainty with which he asserts that the situation will only be made worse by foreign intervention. Will it really? Might not a series of strikes against key targets push Assad closer to the negotiating table, if he sees that suddenly he is in danger of losing? At the very least, it would provide some incentive, or give him something to think about. That could lead to a better situation, as the violence scales down. Or a worse one, if Assad suddenly feels that he is in a race against time to hold on to power (which he would be) and decides to go down all guns blazing. I don't think anyone can predict with absolute confidence what will happen in the event of military intervention. And in the meantime, staying out just allows things to get worse under their own steam.
What a horrible and tragic mess. But in the end, the only thing anyone should care about is what is best for the people caught up in the middle. That is what should guide the diplomacy, even though we know it won't as people get tied down in petty national interest. But one way or another, any action taken that could help the Syrian people while also avoiding military action should certainly be explored fully. I hope it works and makes a difference.