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I think the USB host mode of the Kindle is not explored enough to be a reliable choice. Also, implementing (virtual) USB on the Arduino is a resource-hog... So I would suggest to go via serial. Should not be too hard, given that the Arduino already works on TTL levels. Though the Arduino will probably use another voltage, so you will have to put in voltage conversion in between. For Arduino to Kindle, that could be just some resistors keeping the voltage down, but for the other direction the Kindle's TTL levels (1.8V mostly) will probably be too low and will need a driver. Maybe you can use some cheap spare AND/NAND/NOR chips for that. In combination with USB networking, this is also a very development-friendly solution since you keep shell access to your Kindle for testing.
As for the second task: Not that easy, but not too hard either. Shameless plug: The KindlePDFreader application is in fact a Lua scripting engine which - among other things - can be used for just painting some text and graphics in selectable font (just dump the high level Lua parts of that application and write some bits on your own). Might provide an easy start (because it's scriptable).
If you'd like to go more complex, I think the easiest starting point is Qt embedded (for which graphics output works almost out of the box - a small patch is needed - and input drivers are also floating around on the Web).
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