The way I see it:
The light is 100% Maou. Maou has never aimed for world domination or whatever (made clear when Jahy explained she did all the work in gathering an army, etc.) Maou has always wanted the destruction of the gems and gave original mahou shoujo her powers too (which is why she ended up living with her after)
As for why she's never told Jahy: either Jahy gathering the gems makes destroying them easier, or she just doesn't want to bother with the hassle of explaining.