Realistically, it can only be "backwards compatible" with Gamecube, since all Nintendo systems before that used cartridges. And they're not going to fill up the Revolution with cartridge slots.. it would triple or quadruple the size of the system, which totally defeats one of Revolutions biggest selling points.. it's small size.
However, the rest of the Nintendo library will probably be up for download, stored within the 512MB flash memory, and you can play them at will. The flash memory will basically act as a big cartridge, holding many games. So you get all of the advantages of a cartridge, like zero load times. It will probably hold quite a few games, too, since many cartridge-based games are surprisingly small in file size.
As for your other two questions, it's far too early to tell. Just wait and see like the rest of us.
