It was just a rez, now you can use it as an absorb also. Dark Armor has a lot of other mitigation so it doesn't *need* to have an absorb up 100% of the time.
Do you mean Cloak of Darkness? That defense does not supress. CoF doesn't have defense. However, CoF does have knockback protection, -damage and -tohit (as well as the Fear mentioned in the name)
Depends, I used to take OG and skip CoF. CoF is a lot better now though, so I swapped to taking CoF and skipping OG. Except on things like my DA/Stone tank who stacks AoE stune between OG and Fault.