Summoning Technique and Edo Tensei Technoqie are two different Jutsu/techniques. One is signing a blood contract and the other is utilizing a special charm placed inside the brain to posses the individual being Reincarnated.
OK, let's go through it pointwise.
1. Nup it's not actually under same contract, because contract requires blood activation, and edo tensei requires DNA of the guy you want to reincarnate,
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Note Kabuto takes Torune's blood on his Kunai, places them on his contract, and then starts Edo tensei.
2. But point here is all of them are bounded by same hand seals, and hence when Kabuto released the technique, all of them were released.
3. When Madara did break edo tensei, he became nothing but a summon person, who can control himself, OK, so because Madara cut off that contract(by overcoming the 'effect' of edo tensei, same thing that Itachi did using Amaterasu), but Itachi was still released because Itachi didn't know a way to break 'his edo tensei contract', Madara knew it.
4. Now why Hashi and Minato can't do that?
Because they don't know the contract through which Madara was bound, so they can establish it again, even if they know 'edo tensei'..:yeah:
points 1 and 2 should not apply considering the mechnanics of summoning has changed when edo started summoning creatures
3 and 4 are the points im trying ot make. Madara became his own controller and since he is a summon still the mechanics of how the contract seal works still should apply to him
In theory Edo deidara spams c0.in theory Edo nagato CTs a village every 5 seconds.in theory Edo madara can sit in PS while wood styling anyone into submission..