Off to work so I'll post additional comments later.
Great writing of Lois and Will.
As I suspected it wasn't kryptonite. Superman would be glowing a dark green if it had been and would have aged to his full 60 plus years.
Forgetting magic only 2 other things can lead to his death. Red solar raditation and gold kryptonite. There is only 1 piece of gold K around per canon and Superman has it in a vault. In Whatever happened To The Man of Tommorrow Superman used the gold K to kill himself.
Lois knows what killed him. She knows of gold K and maybe is going to see if it is missing?
As to whom? Family member or close friend is my guess.
As to why? Perhaps a benign reason? Years of kryptonite exposure taking their toll on Superman's body - he is clearly aging which Kryptonians wouldn't under a yellow sun with no K exposure (aging would occur but over hundreds of years). And his mind? Had he become a potential threat to humanity?
