Well, yeah, it's in China's interest to keep the Strait open, unless - of course - Iran decides to close the Strait only for tankers not going to China.

I actually don't think Iran has that capability. See, the only way Iran could effectively close the Strait without the US being able to forcibly re-open it with ease, would be to mine it. Iran has enough sea mines to do that (and funnily enough they bought most of them from China), but they don't have mines that are smart enough to only blow up if the ship that hits them are bound for somewhere other than China.