It should be pretty much the standard way to configure any Linux network (Kindle), or to set up a masquerading router (PC).
Make sure that your PC is correctly configured to forward packets from "internal" (usb0) to "external" (eth0), and back, using NAT. Here's a very compressed howto, use your favorite search engine to find more exhaustive documentation if needed:
http://www.revsys.com/writings/quicktips/nat.html
Then configure the Kindle to use 192.168.15.1 as the default gateway. Finally, I don't remember exactly how DNS is configured on the Kindle, but I'd try the standard way (/etc/resolv.conf) first.
As far as the proxy is concerned: I don't know. There might be a setting buried deep inside the system, or there might not.