Here's why I think the evil tadpole and the evil playthrough are linked and can't be seen as separate stories. And perhaps this could be made clearer for people who approaching this as new video game and not as a successor to the BG story. To me it seem connected because I have the Bhaalspawn story in mind.
The tadpole as Bhaal power analogue. Spoilers only to to avoid wall of text.
So long story short, the motivation for an evil playthrough should the like that Sarevok and Mellisan. What if we could take the power but avoid the end? Don't remove the tadpole, control it. Find a way to use the Bhaal power for yourself, find a way to have the godlike powers of the absolute without becoming one of her slaves. Sure I'm wearing her brand, brandishing her shield, wearing her gloves and shouting her name but I have a card up my sleeve. Like Mellisan, I'm aware.
If I were to side with the goblins I as a sneaky neutral evil \ lawful evil PC I would be thinking "get in good with the real powers at moonrise towers, find out everthing I can about the Absolute and how I take her powers without becoming slave" To do this I want to impress that eye that is hovering near the drow not just Minthara herself.
And this should lead Tav down the road to ruin because that's just what happens in the FR.
From the original FR campaign setting:
"Finally, the Realms are a land of adventure, and therefore adventurers. It is the time of heroes, when one man of pure heart (or with a powerful artifact) may hold his own against enemy hordes, where legions of evil forces may muster and be destroyed by the actions of a few, where the nations rise and fall on magical tides which mere men can control."
Although I would note that Larian has chosen to assume that the cannon ending of Avernus is the failure of good. Zariel is alive and evil as ever.
I want to mainly address the part in red, but I'll open up and say that I have reasons to believe you're right, the Absolute's power and the evil path seem to be linked. I can assume that based on the fact that your tadpole powers only seem to work on people with some connection to the absolute. I can remember ever being able to use them on someone who is not related to the absolute in any way. So perhaps Larian goal here was to make these storyline paths one storyline, but alas, they didn't. I'm not sure if it's bad writing or Larian's insistence on "as many choices as possible" shtick but right now, You can use your power as much as you want and still be a "good guy" (and by that, I mean choosing any variation of the "good path"). As such, I can judge the evil path as if the tadpole powers are a big part of it (but it's not, you can finish the evil path as well without triggering any dream), but even if I did the evil path still doesn't contribute much to this storyline.
I'll try to use what you wrote (the red part) as an example - you wrote "
get in good with the real powers at moonrise towers [I assume it's a mistake because your character doesn't even know the tower exist by that point in the game regardless of your allies]
find out everything I can about the Absolute and how I take her powers without becoming a slave" this sounds great in theory, but there is nothing in the game to suggest the goblins has any answers for these questions. they are fanatic fans sure, but they know even less than you, and if they do know, they don't indicate it nor share it. I think the version of the evil path you are talking about is great, but I didn't see any of it in the actual game.
