Obito had one Sharingan left due to the one he lost during his battle with Konan, so this prompted him to just replace it with one of Nagato's rinnegan. Really, I'm sure even you can do this math.
Well, if he were eternally locked in the Demon's belly (With Hiruzen, I might add.) How would his soul be returned to the human world? Questions we can't answer very well, my friend.
Actually, The Second Hokage made himself a sacrifice for his village. There's a manga chapter, where he does so as well. XD It's when he appoints Hiruzen as Hokage.