kay i think you can explain it quite reasonable:
first of all he showed us a flashback of obitos life, so it would be unlikely, that its not obito.
then the uprising questions:
1.how did obito regenerated so fast
2.how did obtibo obtain so much informations about madaras life
3.how did he know so much about the old world und the uchiha-clan secrets
4.how did he grown so strong in such small amount of time?
5.then who gave nagato his sharingan, cause obito wasnt alive or still very young
6.who controlled the mizukage
7. who did his attitude to life/peace changed
its quite obvious that he need some kind of sensei to achieve all this things, so we can assume that madara was still alive, cause he was the one who gave nagato the rinnegan. so madara and zetsu helped him to regenerate, then he learned all things from madara, and awoke his mangekyou and gets then the eternal mangekyou, okay thats not hard to explain.
okay then the timeline question: he seemed like an adult when he fought the 4th, okay you can say he grew in the time between the fight and his death, but it would be too vague to say that. you can say that his kamui functions like the time chamber in dragonball, the time pass very fast in the other dimension, thats why he can teleport himself, cause he travels the long way in the other dimension but the time doesnt really pass in the real dimension.
then you can say when he awakened his kamui he could train much longer and grow stronger and then he is physically older than the others too.
i think the turning point, which will explain his change of attitude, will be the death of rin. maybe was madara manipulating his mind too, and turned then evil like anakin in star wars.
madara dies, tobi grow his long hair, and takes the role in the world as madara, controlling the mizukage and helping itachi to eliminate the uchihas.
other theories that would work: its all a genjutsu (kishi troll); somebody other than obito is in his body(madara/madaras mind(would be too very plausible), izuna etc.); orochimaru helped him(very unlikely), cause we would have new plot holes.
so guys chill, kishi has all possibilitys to explain things
-> wait for the new chap for some answers
