Global architecture questions

Only a partial reponse to your questions:
Apart from the neccessary administrative data, it is just what happens to boil up in the head of the individual Node designer/manufacturer. So imho unless parties will agree on a voluntary application layer standard of device and command classes, it will be hard to create generic interfaces in third party systems. In some other thread I have suggested to have a look at OpenZwave, which has the same 868Mhz bandwith time constraints.