You are correct. That is how sealing jutsu work within this world. The reanimation jutsu is not a sealing jutsu. Like Kabuto said it is a summoning jutsu. The souls that are sealed atm should not be affected by releasing the summoning jutsu. The souls were simply brought from the plain of the dead to the living plain, then bound to a corpse. Some of those edo's since have had the sould that was bound to it already unbound during the process of the sealing jutsu used on them. The ones Like Sai's brother have already been released on their own which you already spoke about long ago. As far as I know the only way to affect a soul that has been sealed by any means first requires the unsealing version of that seal to be cast first. Orchimaru and Nagato will forever be within Itachi's blade. Unless at some point his soul is called back to the world of living and he has the ability to release that jutsu. Some for those within the reaper. They are there forever unless there is a way to release them from the death gods belly. Releasing the s edo tensei will simply unbind the souls from the corpses and they will move on to the next realm as they did the first time. Unless some other force prevents the soul from moving on like with Kakashi and his father for instance. They were in limbo.