As for "why obito's body"
its possible he/they (maybe tobirama AND sasuke?) simply needed an uchiha for...whatever reason, maybe MJM's idea of wanting the uchiha to be hated or whatever. whether he's being possessed or whether this is edo tensei or why him?
even if they needed an uchiha, they took obito's body during a war. i doubt obito was the only uchiha that died, if he was then...wow. perhaps, if possession and not edo tensei, they needed him to be alive and the other uchiha died too quickly so they couldn't get to them?
hmm it being obito's body just seems weird. i guess its possible this is a collaborated effort and obito is, like, the 3rd tobi. whether or not the others are still around remains to be seen, assuming more than one. but they could have found him and manipulated him or brainwashed him or something to make him hate the leaf. though this requires tem to basically be making a club for people named or who's name can potentially form "tobi" which is dumb as heck.
the only reason i even entertain that thought is because it seems both plausible and stupid, and i still don't have much faith in kishimoto.
thats really the question I have. why obito. at the very least this is clearly obito's body or form. if not possession and its, i dunno, some form of transformation, why? to throw off kakashi? why do that JUST for kakashi. no, transformation seems unlikely since theres little reason to take obito's form. its definitely his body. but WHY???