Even worse for China is if they are blocked from both US and EU markets. Like the US, Europe has found its IP and technology appropriated by China, so with the right strategy Trump might convince EU leaders to also take a hard line against China. Unfortunately Trump's world view doesn't allow for such a strategy.
The big obstacle to this strategy isn't Trump's world view, it's the structure of the EU itself. They just haven't been able to agree on a hard line against any kind of Chinese trade abuses. This has been a problem since well before Trump was elected.
China’s economy is very dependent on foreign markets, both for exports of its manufactured goods and for acquiring the commodities needed for that. Their currency peg further limits their options. If they go too low commodities become unaffordable and if they go too high exports lose competitiveness.
Not likely. When manufacturing comes back to the US it's because the company calculated the cost of a US-based automated factory was less than the shipping + tariffs on the foreign-produced goods. If manual labor jobs leave China, they will go to other low-cost countries like Malaysia, Vietnam, etc. or be lost forever. Even producing things in Mexico can cut your labor cost in half in comparison to producing here.
It could mean more US jobs, but it could also mean fewer if you look more broadly.
Say US companies are having PCBs manufactured in China because that's currently the cheapest option. You cut off that option and US companies are forced use more expensive domestic options. The effects of that can ripple through US companies that directly or indirectly depend on PCBs, and the jobs lost throughout the economy could be greater than the jobs gained from the new automated factories.
More importantly it's a strategic positive. Productive capacity is defense capacity. Buying gewgaws, knickknacks, and flipflops from a strategic competitor is one thing, buying defense technology is another, and it's not the path to security.