I did not read everything above, but here are my 2 cents:
If the evil path gives you a new skill tree with godlike powers but a bad ending (for example losing most companions or turning into a mindflayer) while the good path will give you the good ending it may be interesting. It would be bad if you can get all those powers and still get the good ending with some coins or a skill check. (looking at oathbreaker "You did something really bad but if you give me some coins you can be a devotion paladin again").
In the beginning all mayor quests are about getting rid of the tadpole. I see absolutely no reason to put aditional ones in my brain. In my first EA playthrough I avoided using the tadpole at all and I was asking myself "What is the dream person everyone is talking about?"
My best idea is this: When the full game comes out I will never use the tadpole or use another evil source of power. I see tons of new stuff all the time because I play the full game the first time. If I miss something this way I do not know what I have not experianced so I do not miss the stuff I have missed. (language can be confusing ;-)
If I play the game again I can use this power whenever I want and so I see much new stuff I have not seen before.
One good thing in EA (and hopefully the full game too) was that there was no point in the game where you were required to use the tadpole if you want to progress at all.

Prof. Dr. Dr. Mad S. Tist

World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already