Something you seem to be emphasing is the idea of being able to use these eternal spells near infinitely. I suppose spell effects normally too weak to be worth creating could be made into eternal spells, but 'weaker' spells tend to lend themeselves to situational use, which in turn, renders the purpose of eternal spells useless.
The core mechanic appears to be reusablity, But I find this reuseablity to be an extremely powerful tool, in that even a relativly minor skill of granting target creature +1/+0 could be extremely powerful and dangerous if repeatable.
Perhaps instead of thinking of 'Eternal Spells' as light abilities that are reusable, we should work from the other end, making them very powerful, but perhaps less effecient then their normal counterparts, useful if repeatedly used or needed to last a long time, but otherwise not worth it.
Light Infusation:4

gives all creatures biolumence and heals you for 10 HP.
Reads roughly the same as it's counterpart, but less repeatable. It also may be cheaper in the long run to have a Light Nymph, as that creature's skill would be cheaper then the 'eternal spell' cost.