US President Donald Trump tweeted on Friday that he is ordering US firms to find an alternative to China and bring their business home to America.

Over the weekend, Trump asserted that he holds such broad power thanks to the International Emergency Economic Powers Act of 1977, a piece of legislation signed by President Jimmy Carter.

My questions:

1. What precedent, if any, supports Trump's position that POTUS can use a national emergency declaration to regulate and compel international trade? Or are there any precedents against the view?
2. What is the legal process by which an administration executes that law's provisions?
3. In light of the fact that China contributes billions to the earnings of US corporations like Apple, how could US firms challenge Trump's supposed legal authority if he does execute the order?
4. What can Congress and the courts do to stop Trump from attempting the sweeping order on trade?