I see. Mmh ... well, since we agreed on the "hatching prematurely", maybe the egg hadn't enough venom to cause all those physical reactions aside from the changing of hair colour and the hallucinations? Or Shion recovered from the spots since he's young and all?
Oh, sorry for making you think of that then xD Keep suppressing that thought.
No, probably not. It's about the melanigenous (is it written like that o.O) cells at the hair bulb. So the hair would grow white but wouldn't turn white right away. Hair is dead anyway - it isn't just gonna change colour suddenly xD
Just imagine Shion stopping Elyurias from vanishing and flat out asking: "How do those eggs even work? Like, I understand everything except those scars!" xD Poor Shion.