This is a great point. However, this is where a confirmed timeline by Kishimoto would come in handy. The idea that obito couldn't control the nine-tails makes perfect sense, and yet someone had to have saved obito for him to even be considered alive in any theory, so depending on who saved him, and his development of the MS during the inbetween time, the possibility of controlling the 9tails is a stretch but not impossible.That, and there's NO WAY obito can control the 9 tails, be itachi's sensei, cast a strong enough jutsu to control the mizukage, have space time jutsu greater than the 4th hokage, founded akatsuki and know so much about the uchiha/senjus history and the secrets of the Sage of the 6 paths........oh, and counter kakashi's kamui and sasuke's ametarasu.U_U
For me, that spells madara with a capital 'm'(.....nd he held a grudge against the uchiha clan so would help in its destruction, yup, u get my point).
The space time jutsu is the clincher here, and again something like this would depend on who saved him if it were Obito. We have seen plenty of jutsu be trained in naruto in a span of weeks, such as the rasengan so its possible but highly unlikely and I like this debunk the best.
I haven't seen anything to prove Madara controlled a Mizukage, and if he did, it wasnt necessarly at the time before the ninetails attack and the birth of Naruto.
Knowlegdge of the Uchihas is easily explained in that everyone Uchiha knew about the Uchiha issues against the leaf pretty much except young individuals like sasuke. So the fact that Obito would have that knowledge just as Itachi did is not that much of a stretch.
And knowledge of the SotSP is not show nor is countering amaterasu or kamui until way way later so his skill had plenty of time to increase by then as well.