No, that's actually not true. There were reports that the Syrians had done it a number of times since then, verified by the U.N. and others, and which the Obama Administration itself acknowledged in 2016.
http://www.politico.com/story/2016/08/obama-syria-chemical-weapons-legacy-227460
Depends on the issue. Obama blamed Bush for the economy until virtually the day he left office. But in this case, I blame Obama for making our credibility for shit on the issue of chemical weapons by making threats but not carrying them out. So I blame him, right now, for Assad having and using chemical weapons.
Going forward from this, if Trump ever tells us that the chemicals are gone, and we are then hit, it'll be Trump's fault.