What is the best outcome now, in terms of leveraging Assad to stop using violence?
This option is no longer realistic. He has to leave.
How can China be involved to pressure Assad?
China won’t do that but might offer positive incentives for him to leave by helping him to find a safe haven in some third country.
Besides China, who is the next big influencer here?
Turkey is the one country that has the interest and the capacity to remove Assad but it currently considers the action too risky—they might get sucked into a long civil war and insurgency and become more vulnerable to terrorist attacks.