I don't think loose buttons are a reason to return the device. While popping/cracking noises when you press buttons and rattling volume control buttons when you shake the unit are indications of overall cheesiness, I think that those behaviors fall well within the norm for the device. As much as we may love what the device does and what it represents, I don't think anyone would argue that the physical construction of any of the Kindles leaves something to be desired.
That said, the K3 seems to be an improvement over the K2 mechanically -they got rid of the slippery smooth steel back cover and the less than optimal "joystick". The DXG is, mechanically speaking, an oversized K2 with an upgraded display so it's no surprise that it has many of the K2's faults.
If the buttons don't work, send it back. If the buttons do work but the noise bothers you, you need a different reader, not another Kindle.
When I lived in Japan for a few years I found that things that are acceptable to Americans are often unacceptable to the Japanese. Electronic devices you get in Japan are generally very high quality and priced that way. US consumers seem to place much more emphasis on price, so we get items with rattly buttons which would never be accepted in Japan. It seems many European cultures are similar to the Japanese- they are more sensitive to and value a quality feel more highly than most Americans.