1. Yes, but heavy use of Whispernet from outside the US when registered as a US resident might be one of the ways in which overseas people get caught, along with not having a US IP and CC billing address match up when purchasing.
Some people on this forum have been sent proof of US residence requests from Amazon Customer service, which can be pretty hard to sort out without corresponding documentation, and have had their accounts blocked from further purchasing in the meantime.
2. Sorry, Windows for PC was a typo, and I meant Kindle for PC.
Kindle for PC/Mac/iPhone, etc. all have a separate registration from the physical Kindle device, and is how people without actual Kindles can go and buy/read Kindle books. As long as it's registered to the same Amazon account as your Kindle, you can buy books for the PC version, then go into the Manage Your Kindle page on Amazon, and select Transfer via Computer.
This will let you download a file encoded for your physical Kindle, which you can put on it via USB, all without having to go through Whispernet, if you are worried about being caught through the wireless delivery.
Alternatively, you could always set up and maintain a completely separate "US-based" Amazon account for purchases only and strip the DRM from them as they come in to the Kindle for PC app, and then register your Kindle DX to an address in a neighbouring country which is officially supported, so you have worry-free use of the Whispernet for web browsing and such while still being able to buy and read books from the Kindle store.
And if one day Amazon officially supports the Kindle in Turkey, then you can magically "move" to your real address with all your purchases intact.