But my question is - since you are normally required to add 50ft to the CDA flown NPA minima, why would it not be required here too? Aren't you likely to sink below the MDA if you commence level-off (for circling) AT the MDA?
You can't start to level off at MDA. The airbus procedure is to start levelling off at height 1/10th of the vertical speed at the moment by push to level button, which is appx. 70 to 90ft before the MDA so you level off at circling MDA and well above NPA MDA.