re: Why was it so easy for Trump to get the Sauds to support safe zones?

Maybe. I'm hopeful that there will be two years or less of fighting. At some point Iran and KSA have to discuss things, as most of this conflict is out of the purview of the US, and has references all the way back to Ottoman-Safavid treaties. I'm interested to see what Trump does with the KSA as he seemed antagonistic to them at times, and thawing the Iranians and introducing them to the world market may moderate their regional aims. The only long term solution I see for Syria is a situation like Lebanon with strict ethnic and religious representation embedded into the constitution.