What exactly was the deal with the shitennou once they "reverted" back to their crystal/mineral forms?

I know from having read the manga that they don't figure into the timeline of Crystal Tokyo (IIRC the last time they appear is in the Dreams Arc), so what was their fate there after?

Why did they become minerals/stones upon their deaths whilst remaining sentient? Were they dead or was it more a holographic projection of their consciousness? 

Were their mineral states similar to Mamoru's own Golden Crystal? If so, will that be the fate Mamoru meets upon his own death? If Mamoru were to reincarnate anew, would the shitennou be reborn as well?

The Amazoness Quartet reverted to stones as well when they were defeated, so maybe the stones Mamoru has are actually physical starseeds. They can communicate with Mamoru because of his own emphatic abilities, probably, since he was able to project his own soul in Crystal Tokyo and talk with people. We only saw a very small part of Crystal Tokyo too, so we have no idea where the Shitennou were (and who knows if the future has shifted to include them later...)

So they're not quite dead, but maybe they're just too weak to take human forms. They could possibly regain bodies if they were healed like the Amazoness Quartet was.