It's not that I don't think they're susceptible to manipulation, it's just the kind of manipulation that I'm skeptical of. Like, I don't see this angle ever believably working against Lae'zel because it's all stuff that's anathema to her specifically. she already assumes its the tadpole and that accepting it would be anathema to everything she believes.

Also, I assumed Shadowheart was dreaming about Shar and just not admitting it, not that she was dreaming about a friend or lover. Presumably her dream would be about becoming a Dark Justiciar and Shar telling her the power of the tadpole could help her reach that. Which could work if, again, she didn't already know the dream was a lie. She also can become a dark justiciar through her own quest with no extra help from the tadpole.

Presumably Gale dreamt of Mystra. Maybe he dreamed that the tadpole could help him ascend to reach her and win her back? That could maybe work, if it included dealing with the orb as part of the deal. Fair enough.

Again, Wyll seems to be dreaming of Mizora, who he is still talking to in real life. Maybe being offered a way out of his bargain with her could work, but just posing as Mizora wouldn't help with that.

Karlach meanwhile? She would probably dream of either Zariel or Gortash. She never wants to deal with Zariel again, and she specifically wants to kill Gortash, someone she thinks she's more than able to kill since she thinks he's just a corrupt politician, not the head of some dark cult.

I agree that Larian wanted us to use the tadpole powers because they think it would be cool, but the more I think about it, the more I think Daisy and the associated stuff just wouldn't have been able to hold up past the first act anyway.