I mean....the Darkness did cripple the Traveler itself, so yeah it did that and can do much more. The Traveler/Light created the Ghosts. So yeah...it did that.

They can effect without cause. They can cause without effect.

Technically we should be able to cast spells like a mage from any rpg but all we get is a super, grenades, melee, and a class power. Just replace a wand or staff with a gun and that's a Lightbearer, we are space-mages whose object to channel their power is a weapon instead of a stick.

Archival Mind is right. The light and dark are not bound by any kind of universal law or constraint, as such they and those that use them can do pretty much anything, event thing that violate all laws of the universe. That is why the thought that the light and darkness are some kind of universal balance is stupid, the balance is just some primitive theory that half clever beings who adhere to the laws within the universe project on powers beyond the universe, if we can let go of those in-universe-box way of thinking we would be able to do so much.