On the subject of summons dying with their master, Enma didn't die when Sarutobi died.
And thinking back to the flashback when Itachi explained to Sasuke why he killed the clan, he said something to the effect of to test the limits of this container. Perhaps Madara does have a soul transfer skill like Oro. It could be possible he took over Itachi and transferred over to Obito later on (my theory about how Obito/Akatsuki). If the latter is true, it could also expalin Tobi's split personalities.