A foreman immediately roused 8,000 workers inside the company’s dormitories, according to the executive. Each employee was given a biscuit and a cup of tea, guided to a workstation and within half an hour started a 12-hour shift fitting glass screens into beveled frames. Within 96 hours, the plant was producing over 10,000 iPhones a day. “The speed and flexibility is breathtaking,” the executive said. “There’s no American plant that can match that.”
“They could hire 3,000 people overnight,” said Jennifer Rigoni, who was Apple’s worldwide supply demand manager until 2010, but declined to discuss specifics of her work. “What U.S. plant can find 3,000 people overnight and convince them to live in dorms?”
“Another critical advantage for Apple was that China provided engineers at a scale the United States could not match. Apple’s executives had estimated that about 8,700 industrial engineers were needed to oversee and guide the 200,000 assembly-line workers eventually involved in manufacturing iPhones. The company’s analysts had forecast it would take as long as nine months to find that many qualified engineers in the United States.”
So no it wouldn't be possible for Apple to move production to the United States and get the same production and flexibility that they do in China for a couple dollars more in labor costs per unit. I would say it is not even possible to get near the same level.
"Operators then may want to customize the software, and the OS must be localized for the market and language."
I think that is where the bulk of the time is spent.