The reason is obvious: external elements are not internal -)Yes, though there isn't a consensus on the reason for the distinction.
Yes, because this is also a samsaric realm. I just said that, according to him, the perception is best over there, in this "sphere of nothingness". Don't ask me how so and why - it is just written in the suttas.I am under the impression that the Buddha let go of the sphere of nothingness.