Larian called a big retreat when it came to consequences and reduced a lot of stuff in this regard. Tapole Power for sure was always something they thought of being an important decision and I am pretty sure they would have followed it through to a point of no return. For example not being able to refuse turning into a mindflayer and making checks to resist harder and harder. I believe a lot of that got recycled into the Dark Urge Origin as the whole tad pole thing, like sedurctress, didn't revceive a lot of positive feedback in EA. Instead we got something which shouldn't anger to much people and very important don't cause frustration when playing the game.
I for once was really disappointed when i found out that it did not matter if you chose Underdark or Mountain Pass as you could visit both anyways even after entering the Shadow Curse. I am pretty sure this was also changed quite late. As I got some comment from Shadowheart triggering about "Dark Justiciars like in the Underdark Ruins" although until that point I've never been there before visiting the cursed lands.
Hard to design system? I think a lot of rpgs already worked with similar systems to effect the game with such a system. The obvious one first, allignment charts changing through your decisions and in order open up for more decisions in that regard. A corruption system in the direction of Knight of the Old Republic 2 would also present itself. By using your tadpole power more and more you also can corrupt your followers making them follow your dark decent. Giving your main an aura where you can strenghen a certain aspect of the group while neglecting another.
