All of these are rough ideas. And I went on to say that you would have to adjust potency to compensate. Also keep in mind that this would be a rework so the current way to play dark knight would be invalid. Just like how you used to play machinist and summoner is invalid today.

And yes TBN would not cost MP. That's why I said it's now on a cooldown.

How would adding a shield make healing pointless? You can't spam the shield and it would not stack on itself. (Like how all shield works) and I think it would be reasonable to say that it can stack with other shield abilities.