They will work together as long as Obito is alive so Madara most probably wont undo ET.
So there you go Kabuto takes it. Lol
Madara would be doing what he wants, Kabuto never showed being able to control Madara just like how Orochimaru with Senju DNA couldn't control Hashirama. Madara could just kill Kabuto so he can undo the seal

And no one but Kabuto have a means of hurting Juubito since only he had Senjutsu.
He can use C0 with Sennin power( I wonder about that Lol)
Madara's meteors
KA from Itachi
Izanami from Itachi
Nagato takes all ninjutsu/tbb since he is Edo and a lot stronger than 6pop
The list go one and they can create a strategy against him, while Obito is all alone.
Meteors wouldn't do anything since it's not Senjutsu in fused, he'll probably break it as he did the seal Hashirama put on him and this is what was holding down the Juubi
You must be registered for see links
.
Naruto had KA, not Itachi. Itachi gave Naruto it so it was Naruto's power and if you say it's not then it's not Itachi's since Shisui gave him it.
Nagato's jutsu to absorb ninjutsus is a ninjutsu, therefore Juubito can attack and cancel Nagato's absorbing jutsu. Stronger than the 6pop? That was healthy Nagato who absorbed Bee's Bijuu chakra, he wouldn't be healthy in this senerio since he's just summoned by Kabuto.
Juubito summon God tree which starts to absorb everyone's chakra, including Kabuto's.