2.Is the API for the JSON/REST backend documented anywhere?
It is awesome to see someone implement this architecture.
I have been working on a similar concept for quite a while, but didn't have time to build anything more that prototypes and proof-of-concepts.
2. We're passing JSON-LD (http://json-ld.org/) over the standard Backbone.sync API (http://documentcloud.github.com/backbone/#Sync). But for other systems that can be replaced with a different JS implementation to the fit the API conventions of the back-end you talk with. Just override Backbone.sync
If you're working on something similar, then our JS libs will probably be handy: https://github.com/bergie/VIE