Is China really fully behind helping Iran? if why would it be actively undermining Iran's strategy imho ... Had China carried on buying oil on the international markets, the economic affects would have more of a bearing on the US decision making process than it has done. China provided the USA with a get out of jail card with its approach.
I think what China wants, is not for Iran to win or lose, but for the USA to get dragged into a forever war that saps dry the USA's military and economic strength, and I don't think they mind if that leads to further destruction of Iran, her economy and her military as long as it hurts the USA even more.