You're right on many accounts, but the last remaining snake wasn't sealed. It was burnt by Amaterasu, meaning that a part of Oro was only killed, not sealed.
That might play a role in the future, who knows? :shrug:
this is my point............ yea i was wrong about jugo sorry....... but i wasnt wrong about oro thank youEven though Oro was sealed, one snake escaped and was only killed by amaterasu, thus implying that not all of Orochimaru's soul was sealed and therefore leaving it possibly open for him to make a sensational return.
Also, there are more secrets to Edo Tensei that Kabuto has not revealed.