Madara is no longer Indra.
But then exactly who is he?
I speculate that he is a nobody. That is why he is a soul stuck in Limbo.
But at the same time, being in the Limbo World grants its perks, like being able to attack in the real world.
The real question: Is the Madara in Limbo the real Madara? Or are they both real?
And please don't give me this crap that Transmigration is just Chakra being passed on. Kishi wouldn't use such an important word like Transmigration to describe basic chakra transfers.