Leave postdoc in the middle of contract to start work with another professor while being in one postdoctoral research
You should be as up front about the situation as you can --- with both parties. Prof. A needs to know that you are interested in a one-year deal but cannot currently commit to anything longer. Prof. B should know that you are interested but have another opportunity and will need to know about the funding. Worst case situation, you join Prof. B's team 12 months down the road. By that time he should know about the funding.

As to #1, you answered this yourself. Once you commit yourself for one year, you should stick it out. Of course it is always possible to get out of any contract, but at what cost? Seems that it would not be worth your while to antagonize Prof. A. What is not clear to me is who makes the decision to extend. If Prof. A does not want to let you go after 1 year, are you stuck?

As to #2 and 3, forget about it. Once you go to China, you will be there for 12 months, and you will work on Prof. A's project. Everything else is unrealistic.