1. you dont need both eyes (2 Sharingan) for Susano'o - Madara performed it with no eyes, and Sasuke has been using it with 1 Sharingan . . .
2. Danzo had Izinagi, but his Sharingan was kept secret. even so, Itachi had in depth knowledge of how it works, so Izinagi being unique to one Uchicha...