Edo Tensei doesn't end even if the user is dead. The user himself has to end the technique or the summons will stay intact until they're sealed, free (soul released) or released by the user himself.
Itachi did say that Tobi is just a shell of his former self. We don't know what he meant by that other than Tobi being a weak version of the actual Madara Uchiha.
Madara was known as the strongest Uchiha of his time with his Eternal Mangekyou Sharingan and all. Not sure if Itachi can go toe to toe with him but we'll see...
