1. BACK_LEFT and BACK_RIGHT are intended to be included in WebGLRenderingContextBase. Note the disclaimer at the top: “These changes are written here in the extension document in order to facilitate discussion of the proposal. They are intended to be merged to the core WebGL specification and removed from here at a later date.”
2. drawBuffer is intended to be included in WebGLRenderingContextBase.
3. drawBuffer() and drawBuffers() are separate entry points, they don’t have any special relationship just because they are named similarly. The similar naming is a bit unfortunate, but it’s inherited from desktop GL.
4. See 5.
5. The current extension proposal is not compatible with WebGL 1.0, though the proposed core spec changes apply to WebGL 1.0. If a WebGL 1.0 version of the multiview extension is desired, it might make the most sense to specify that separately from the WebGL 2.0 extension. Doing that only makes sense after there’s an agreement on what goes to the core spec, though.
6. Including a very simple API usage example is a good idea, but I don’t think the extension document needs to go beyond that. The spec document is not a tutorial.
7. The interaction with WebVR needs to be specified in the WebVR spec. So far this has been discussed only informally.
I’m working on fixing some of the smaller omissions you pointed out. The organization of the extension document is controlled by the XML transform for extensions, unfortunately that isn’t working that great with the current state of the document which includes the core spec changes.