No sane company wants to work that way. Just in Time is a great thing, you shouldn't be over nighting anything you should be working so that you know when you need each part months in advance. If you can get a part in China faster it is because someone has expensive inventory and that is a bad sign in general. You do of course need some emergency supply and such, and retail customers don't plan in advance well - but a business shouldn't be buying retail anyway.
It's not JIT, it's that planning works on months advance notice like you mention is in large part because the supply chain is global and shipping takes a lot of time. When digikey has a month lead time because some component is out of stock, it often means "It's going to take a month for this to arrive from the factory".
In China, you can just go down the street to the factory.
As for shipping overnight, it's incredibly common in R&D and repair.
Only if the factory in China will make that thing today. If they are one their New Year holiday (I forget what they call it) you won't get anything. If they are busy making something else you won't get anything - unless by human factors you can convince them to work for you instead of meeting their promises to other customers.
Sure, but you don't have to wait 6 weeks for a cargo ship.
The concept of having centralized full-chain production for enormous productivity boosts is not some wild concept, it's ancient and well known. Industries have clusters because it benefits everyone in the cluster. The US has very few and weak manufacturing clusters.
In China they have the manufacturer down the street from the distributor who is down the street from your factory.
With Digikey we are paying $80 to overnight a $5 part from Minnesota. We need to be able to go send a guy to pick it up in an hour.