Was he also in the nuclear explosion in the deep space?

Lol. +100 for dat reference. To bad zero is dead for sure now =(

Just a common shapeshifters trope. Powers stop working then you die. Could be different here but I doubt it.

Unlikely, though not impossible)

Just as I said. He could be the first Sovereign aka Force Majeure.

He is brother of Investors. They are shapshifters just as current Sovereign. Also they know how to run the Guild as Killinger knows too. Also, previous Sovereign was called Force Majeure, and asteroid base name is Meteor Majeure (which killinger knew about).

Sovereign is not dead. Eagle was not S. He would turn into it's original form if it died.