Nope. Windows RT is ARM compiled Windows 8 with a little flag that prevents the installation of 3rd party desktop apps. It's not even a little bit different.

Technically speaking, the little flag does not prevent installation of third-party desktop apps. It prevents executing executables not signed by Microsoft. It is just a policy, that Microsoft will not sign classic win32 executables.

Not really. As far as I know, Windows RT is essentially a port of the NT kernel, most Windows APIs and the new Metro/RT APIs to ARM.