Glad to hear that you like the format.
I'm also concerned that the format will undergo change over time that could break previous functionality. Not sure how valid that concern it though.
You don't need to worry about that. I absolutely, positively won't be breaking backward compatibility with any future additions to the OpenGEX format. The current spec is extremely solid and stable.
2) The exporters seem pretty solid, and maintained by Eric, but if I recall, there are some slight differences in support for the exporters between 3D modeling packages. I remember the Blender version not supporting LODs for meshes, which is probably more of a Blender thing than an OpenGEX thing. Not really necessary for me, but there might be other support Blender lacks as that's probably what we'll use.
The Blender exporter supports everything that the Max and Maya exporters support. None of them actually have LOD support because it's not something that's generally available (or done well) in the modeling software itself.