Although you covered this a little, I would rather believe that Madara (without susano'o) created the large rocks in space (not literally outer space, but giving spatial existence), and then pulled it down using shinra tensei (which attracts anything to the user), all using the rinnegans special capabilities. The fact that Madara had to sacrifice himself instead of throwing the giant rock just out of range of him proves his limited control over the art of creating cosmic rocks and manipulating/utilizing them to attack his foes in close proximity.
Edit: Although the Susano'O do form hand signs, I imagine that it is simply an extension of Madara's chakra system manipulation. Unless you can prove that Susano'O also has a chakra system from which to draw on and manipulate with hand seals. Aside from this, there doesn't seem to be conclusive proof or sufficient reasons to believe that Susaso'O is a necessary component to using the art of Shattered Heavens.
Finally, as to why Susano'O was summoned at all during the technique process can be passed off as Kishimoto just wanting to add a greater effect to the technique, not necessarily a needed component for said technique.
