The Story HD is a fine ereader but you should be aware that it can not compete with mainstream models which now come with built in touchscreen and light (Kindle PaperWhite, Kobo Glo & others).

No.

At least, not directly. The iriver Story HD supports Adobe DRM / ADE, so you can shop at any store that interfaces with Adobe technology, and put those books on your reader. In such a case you can use almost any ereader, except for Kindle (which only supports Amazon, not Adobe DRM). You buy the book in any store (either DRM free or Adobe), then transfer the book via USB to your ereader.

Unfortunately I do not know if Kyobo supports Adobe, and allows sales outside of Korea. If it does it would be the best option by far as you'd have pretty much free choice of eReaders.

No.

At least, that's how it works for the German version of the Story HD. It removes the Google books functionality entirely and replaces it with a local German ebook distributor. Chances are good that it works the same for Korea with Kyobo.

Furthermore, the German version of the Story HD Wifi, is sold directly by the book distributor. The devices are registered (each with its own unique MAC adress / identity file). So if you took an US reader or Korean reader and somehow managed to put the German firmware on that, you still could not access the German ebook store because your device lacks the necessary registration data. Again, I can only assume that the Korean device may be locked in a similar fashion.

Switching firmwares around is dangerous; the device bricks easily, even using official firmware files from iriver homepage. This is because they offer firmware "patches" for specific firmware versions, and there is no safeguard whatsoever that could prevent you to install a patch for a version that is not on your device. Incompatible versions may brick the reader and there is no known way to recover. So switching the US device to a Korean firmware (and vice versa) is not really an option either.