A patent is a set of exclusive rights granted by a sovereign state to an inventor or assignee for a limited period of time in exchange for detailed public disclosure of an invention. （from Wiki）
1. 管不到
1A US patent has no exclusive right in China, so unless a Chinese equivalent patent is issued, you can sell the patented product in China.
1B you will have risk selling to US market in violation of patent. But you can apply improved/modified patent to protect different/improved features on your product and seek opporutnity in the US market. The same is to EU. You can also ask the US patent holder to grant you a license to practice in US and you pay a license fee for that in order to commercialize the product based on the previous patented idea.
2. Your choice, you do the cost/benefit analysis and decide how to proceed.
3. I would say yes to apply in US, EU and China. No body can prevent you from violating a patent. But the patent holder can sue you for damage to recoup the lost profit in court later.
4. see no 2 above.
You need a lawyer to help you on this. You knowledge is insufficient to deal with these
