Yeah....I don't think that's how any of this works. You can release the jutsu if you know the seals, sure, but use it? Furthermore, him not needing sacrifices is also assumption, unless its been stated for certain that the sacrifice's sole purpose is to bring the soul into a body.
Dude he was Orochimaru's student and witnessed him casting the jutsu, of course he knows how to use Edo Tensei.
Not assumption, Outer Path puts Rinnegan users outside the cycle of life and death hence why they can control souls as shown with Human path and Naraka Path.
Tobirama's ability to invent Edo Tensei comes from him being the descendant of someone who possessed the power to control souls.
Hagoromo was able to summon the Kages without a sacrifice, and Sasuke can simply dig up some graves if he wants to put the souls in physical bodies.