Here are the answer

If you enter Canada from China, most likely,there will be a record stamped on your passport(ask somebody who has the experience), then 30 days policy is not applicable to your case--you have to stay in Canada in the 30 days.

Please reference http://www.isso.cornell.edu/immigration/autoreval.html

But if you applied a visa in US consulate in Canada, then you can go, but you have to make sure you can get it, otherwise you have to stay there.

If you a Canada PR, things could be a little bit easier.

You have to be very careful.

Good Luck