First why do you need a quad-tree and second why do you need a scene graph and a quad tree in the same system?
I could possibly see using a scene graph or possibly an octree for some lighting effects such as multiple point lights. Usually a quad tree is best suited for terrain rendering and not well suited for indoor rendering. For indoor rendering you will want to use BSP trees combined with portals to get good performance.
Now on to your questions:
I have created several types of quad-trees and my past threads are all over this forum so if you do a search you will find plenty of info. Quad-trees are essentially:
The AABB is the min and max points of the volume that the quad tree node comprises.
1. If a node is completely in the frustum so are all of it's children and the frustum check need not be performed on any of them.
2. If a node is partially in you need to continue the frustum check down to its children until 1 or 3 occurs.
3. If a node is out so are all of it's children and the entire node and it's children can be rejected.
There are many articles about how to create a frustum both for D3D and OpenGL. Just google for it and you should find info aplenty.
In order to accomodate moving objects in your system you will have to have some neighbor information for each node. As an object passes a certain boundary of a node it now belongs to whichever node it is in. For half in and half out you can just select one or the other neighbor. The rendering still procedes as normal with each node being rendered (and all of it's objects) if it is in or partially in the frustum.