1. The only confirmed way to explain Susano'o is that is the result of a double Mangekyo.
2. Obito states that its a rarity for Uchiha to unlock the Susano'o, however, the rarity in itself is the double Mangekyo. Obito himself lacking it along with others like Shisui who lost their eye.
3. Sasuke coincidentally had powers very closely related to Itachi, it would not be a stretch to say that brothers have near identical Mangekyo Sharingan powers. Madara having Susano'o would point to Izuna having it.
4. Tobirama, being a Senju with immense chakra reserves, likely outlasted Izuna and the slash was the final blow to him. Another point adding to this is the fact that Izuna's Sharingan wasn't even activated when he hit by the Hiraishin Giri.
