Why are you comparing oblation to the other tank's 25s CDs? TBN is your 25s CD and it has a shorter CD due to it having an MP cost. TBN having both an MP cost and a cooldown is fine IMO. It makes DRK unique compared to the other tanks and prevents the job from being completely braindead. IMO you should get bonus damage to the free edge/flood of shadows that you get when your TBN breaks though.

"MP management" isn't exactly some fickle art. You just dump edge of shadows into raid buffs and hold 3K for TBN if needed. Spend to avoid overcapping otherwise. You aren't supposed to have infinite MP. It isn't like PLD where you need a constant supply of MP to be able to do your GCD rotation. I don't understand why you're comparing DRK to PLD in this respect.

DRK has comparable sustain to GNB. It's more than enough to keep you up in a dungeon, you just can't solo like WAR can. I'm not sure why you're complaining that your invuln is good? Why do you have it if not to use it? DRK is fine in dungeons.

DRK does need potency buffs. It lost potency via some of the Dawntrail changes and didn't really get anything new to compensate for it. I imagine there will be potency buffs for DRK soon.