I'm going to put all the GUI and Python issues aside for a moment, and share a few things I've learned about creating a non-linear video editor using the Gstreamer framework. One of the original authors of Gstreamer also created a library called gnonlin (i.e. Gstreamer Non-Linear). This library assists people like me in using the Gstreamer framework to create original compositions / sequences... whether it be audio, video, or both. I found a great tutorial of gnonlin with a few code snippets to help me (and you) get started.

There is also a great introduction to gstreamer I found which is full of illustrations. The illustrations demonstrate the various concepts used in gnonln (and gstreamer):