Campden tablets stun the yeast so if there is sugar tree and I they are still properly active this will not work. Plus when you bottle with fresh they will ferment the residual sugar. You could try campden tablets and sorbate but then you can't bottle condition and will have to keg.
I think your best options would be to ferment out and try again with the next batch using a higher mash temp, make another stout and blend them together or possibly use lactose in the fully fermented beer to add back the body and a creaminess to take the edge off the roast bitterness if it's too much.