I believe there is a good chance tobi may not be a uchia at all. When you look at all the uchia that have been powerful(Saske, Itachi, Madra) they all have one thing in common that those who just have the those the just have the the uchias eye(kikashi, danzo) don't and that is the susano. With tobi being as strong as he is and him being in the stinky situation that he's in it seems as if he would have activated it by now if he could. That seems to be the one thing that only a true uchia could do. So if tobi was obito and especially Izuna since he was at the same level as madara he would have an susano. That's just my thoughts on it.
So first I will make a "prove", though tobi has lied so much what he says may not be reliable, however, he did indeed tell itachi that he's an uchiha:
You must be registered for see links
And as for the susano. There is a reason he can't use susano, it requires 2 MS from SAME person to activate susano.
Although he has the rinnengan, it would seem he can't turn them back to madaras EMS. Also it seems that he has chosen to use Obitos eye allowing him to use t/s jutsu, which would prove why he can't use susano. However, if he indeed is an uchiha I would believe he could use it on madaras level, if he had rinnengan in both eyes. He might even switch to that, now that he has realised he can't win this way, but that would also mean he can't escape as easy.
Another possebility is that you can only use susano with your own 2 sharingans and ur own body, which could prove that he isn't Izuna as well.