My Nintendo ID account is registered in Ireland. Of course, i've never been in Ireland and
live very far away from it. I had to do that because my country is simply not available as an option.
Generally, you can change your countries inside your zone rather freely. I remember how i basically hopped across all European eShops trying to find the cheapest price for NSMB2 last year (eventually i stopped at Greece).
If I were to buy an American WiiU or 3DS can I access the American eshop to buy games if I am in South Korea?
As far as i understand, you can access any regional eShop freely from wherever you are. There are many people with japanese 3DS's around here who can attest to that.
There might be some problems with paying with credit card if address doesn't match though. I personally never had any problems with my local CC, but i know people who can't put money into their US accounts. They have to resort to point cards.