They don't really have an agenda in this scenario. Astarion and Minthara are no more special than any other character honestly. I'm talking about being forced to change, because they used tadpoles. If you don't give Astarion or Minthara tadpoles, then it is different. But I mainly use those two as tadpole slurpers, because I myself don't want to use them.
I meant that characters have their own personalities and if Orpheus forces someone to become illithid, it doesn't make them an obedient zombie, the illithid retains their personality (we see this in the example of Karlach, who changed a bit, but retained many aspects of her personality). Those who have been forcibly made illithid will naturally take revenge, because all the bad things have already happened and they have nothing to lose (rather the opposite, becoming an Absolute will be better than just being an illithid) or they themselves will want to take more power. But after thinking about it, I think it doesn't make sense for another reason - the number of tadpoles is not important, one tadpole is enough for Orpheus to trigger ceremorphosis. One tadpole already triggers full ceremorphosis, Orpheus (in the prism) has blocked signals from the Elder Brain before, all he has to do is stop doing that with any companion or Tav, it doesn't matter how many tadpoles they have in their head, ten or one.
In the scenario of the game, everything is logical and correct - Orpheus turns to the leader of the group. Even if, he could force anyone to change, but he is well aware of the risks. Only someone who volunteers for this sacrifice can be trusted to do so. A lot depends on the character who becomes an illithid, they will have the Crown of Karsus in their hands. Tav can become an Illithid and take over the Brain without any problems, but a companion can't? If Tav refuses (and Tav represents the whole group in this case), and can also talk Karlach out of doing it, then Orpheus has to take over. If Orpheus were a character capable of such a not only cruel but also extremely reckless and stupid (to achieve his goal, and his goal is to stop the Grand Design, it's more important to him than his own life) act, as forcibly changing someone, then among other things, Tav should have had the option to attack him and try to kill him, to prevent him from doing so.
But there could be more volunteers, not just Karlach. Lae'zel, too, could probably offer herself instead of Prince Orpheus, she's motivated to do so. And someone like Wyll or Minsc, Tav could try to persuade (persuading, even if manipulative, is not the same as forcing, the persuaded have agreed to it, accepted it as the right option, and will act as they are required to - to save the world, like Karlach, not destroy it, as the forcibly changed will do).
