We can speculate that Hidan had a jutsu to prevent him from dying of stuff like decapitation.
It was necessary for his fighting style that he could survive severe injuries.
There is no point in stabbing your own heart to kill the enemy if you yourself don't survive it.
Obito didn't have such a jutsu. So, being hit by a large boulder should kill him.
And if not the first, then the several other boulders that landed on top of him.
The reason that he could talk after being hit is real simple. There were just some things that needed to be said by him in that situation and he had to give Kakashi reason.
That's all there is to it.
That he didn't die instantly was by no means a sign that his injuries were not that bad after all.
Rin was a med (she did the eye transplant after all), she should know if he was dying or not.
And Yondaime appeared shortly after he got crushed by the other boulders.
I think he would've checked the place out.
But Obito will probably be Tobi. It seems obvious.
And it's all for Kakashi's sake. So that it'll be a dramatic face-off.
I don't really see why Kishi needs to do that.
Sure, Kakshi is obsessed with Obito. But only after he died and because he thinks it was his fault.
Not because they were such good pals.
Obito was secretly admiring Kakshi, but Kakshi didn't care about him, until they went to save Rin.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"