The fact that the plastic is turning yellow even if you dont smoke, says clearly that they used a cheap plastic.
Im a screenprinter and at work were printing mainly on 2 plastics: PVC and polystyrene. Polystyrene is the cheaper one which will trun yellow just from the sun whereas PVC stays white much longer. (And you can clean PVC with solvents, if you try to clean polystyrene with it, it will melt as the amiga case does).
But i dont think you can blame commodore for using cheap plastics: Nearly all PCs and consoles are made of polystyrene or similar.
So apparently the only thing you can do is not to smoke and keep your amiga away from the sun, but it WILL turn yellow sooner or later.