And if automation were cheaper than Chinese labor, business would have already gone that route
Well, businesses are going that route, but it's a long process. I think it's pretty obvious that the speed is directly proportional to the potential market. Look at restaurants... when activists and politicians started making noise about a $15 minimum wage nationwide, in a short time you got development of ordering/checkout tablets at restaurants, even though the minimum wage hike hasn't happened outside of a few places. Restaurants could have automated 10 years ago.. the technology was there. It just wasn't packaged up as an off the shelf product for restaurants. So Red Lobster would have had to say "Hey Company X, could you develop this for us?" and it would have cost a million bucks. Now, the market is there waiting for innovators, so Company X went ahead and did the development for free and lots of restaurants are buying it.
If an industry was facing a mandate to move back to the US or face huge tariffs, you'd see a big increase in investment towards automation for the same reason... right now there's no market for "machine to automatically manufacture hammers", but if there are suddenly 20 hammer manufacturers that come back to the US, there will be.
That means not all of the jobs come back, obviously... but if we can take 10 million Chinese jobs and trade them in for 1 million more expensive US jobs (so 90% automation), that's still a big win for our economy.
They will not be competitive in an international market where folks can continue to get cheaply made items from other countries.
It's not quite that simple.. if you want an iPhone you have to buy it from Apple, you can't say "Oh I'll get this cheaper iPhone from China." But yeah, for generic products you're right. Theoretically that will result in devaluing the US dollar to make our goods more competitive. Right now you have countries like China that have a massive trade surplus with us, but they take any excess dollars and just invest them in Treasurys, taking them off the market, which props up our currency. In a trade war that would go away... even if they didn't redeem their existing assets they would at least stop.
There won't be a lot of motivation left for anyone to sell us the raw materials we need to make any of our high quality goods
You'll have to elaborate on that. I don't get it. Why would Peru stop exporting zinc to us because we stopped buying Chinese goods?