Abstracting out interface and component linking to XML is very similar to what libglade does already. The fact that one is not connecting the interface via signals allows it to be cross-platform. The fact that one is connecting it via network calls, allows applications to be served over an intranet or the Internet. More information is on our products and developer pages. Now, for your specific questions.
Developers have access to the lower level stuff.
The XML pages call components. Currently, only SOAP components, but Bonobo and XML-RPC support are in the immediate queue. Each program is a set of XML pages in the same way that a web site is a set of HTML pages. Yes, widget creation calls are sent over the network. You can't assume that all apps will load interfaces from XML files.