I don't think the point of Daisy was to convince all of your companions, it was just to provide enough to work with whatever the player chose. Eg. Laezel might seem strong in her beliefs, but you can convince her easily enough not to kill you even when she thinks they are all turning. She is a gith, but also young and desperate to be able to return to her people. And well, players have left feedback in the EA on how to improve Daisy.

The guardian/ emperor has the same issues as Daisy (too obvious), but also no tadpole reactivity. If anything, he was even less convincing when playing Gale's origin, because Gale is so focused on Mystra and his own goals, that this generic approach works even less.

Whether Daisy would have worked better depends on the original concept, becauseI don't think she was supposed to be an illithid inhabiting the prism. But at least in the EA you could use Omeluums ring or avoid Daisy altogether. You cannot avoid the emperor completely, and frankly my impression in the end was that this reinvention of the original legend was just for the shock value of 'it was Baldur all along!'. Ironically, the rewrite resulted in a main plot that doesn't hold up anyway, if you look at it too close.