Use the builtin cssrewrite filter which will transparently fix url() instructions in CSS files on the fly.
See CSS compilers for how this is best done.
Yes. Due to the way Google App Engine works (static files are stored on separate servers), you need to build your assets locally, possibly using one of the management commands provided for your preferred framework, and then deploy them.
In production mode, you need to disable the Environment.auto_build setting.
For URL expiry functionality, you need to use a manifest that holds version information. See URL Expiry (cache busting).
There is a barebone Google App Engine example in the examples/appengine/ folder.