1. Limit breaking your Light Kaiser only gives you more stats and 10% Dark Resist. Limit breaking your Dark Kaiser gives you more stats and 10% Elem+. Getting yourself a Wind Kaiser gets you 30% Elem+ and 30% Thunder resist, at a small loss stats.

Getting the first Kaiser should always be the priority, at least for elements that you care for, IMO.

2. Can't comment, my knowledge of Light is awful. RNG has not given me jack-fooking-shit for Light, so.

3. Whatever you care for the most, really. Put it into the element that you like playing.