1. Each node generates its own private/public key pairs, but it's also possible to share these across nodes if you really want to, using the dumpprivkey and importprivkey commands.
2. MultiChain uses a model of consensual administration, where one or more parties act as administrators on the chain, and these administrators together decide on permissions for other addresses. For more information please read the white paper or the permissions management page:
http://www.multichain.com/white-paper/
http://www.multichain.com/developers/permissions-management/