You make a lot of good points. Maybe the lack of companions pulling for the cult path will change when more are introduced, I just find that odd since these are supposedly the evil companions that we have (or just evilest, since I don't think Gale and Wyll are all that evil, and to my mind, the jury is still out on Shadowheart being all that evil).
As for your comment on character ambitions, my problem there isn't your character thinking you can outplay the cult, it's that what you stand to gain by outplaying them isn't at all clear. It doesn't entirely feel like you get to learn much about them until you're already there in their base doing stuff, and I'm not entirely sure how you could reasonably get there unless you'd already agreed to go find Halsin. Otherwise it seems more like a place you'd just steer clear of. Maybe if they tweaked that meeting with the True Soul and his followers from early on so that when you and he meld minds, you get a better sense of the cult, that would make more sense to me. I wouldn't want that mind meld to give you all the information, but having that be when you learn that True souls are respected in the cult, maybe get a vague sense of how far-reaching it is, and where their base is, that would give clearer impetus to go there. As it stands, you don't actually learn much from that encounter beyond the fact that the cult exists in some form. Based on what you see, you don't have much reason to believe the group is meaningfully organized or powerful or a thing you can join. Basically you don't have much reason to seek them out and try to join them until you're already there, no reason to think that infiltrating and trying to take them over would be an excercise worth your time. It's all well and good to leave room for the player's imagination, but if you leave too much to the player's imagination then it's not going to be a coherent, cohesive story anymore.
This also makes your comparison to the lack of guarantee with the Halsin quest weaker, because it's not as though you ever reach a moment where you have to stop and think "do I try and find Halsin or go with the cult?" It's more like "I could try and find Halsin. Also th goblins that took him turn out to be part of a weird cult I don't get any opportunity to learn much about."
You make some good points about why some characters would join the cult, but most of your points hinge on the assumption that the cult can actually do something about the tadpoles. My problem with that assumption is that no one in the cult knows they've got tadpoles. So there's no way to know if they can actually control it, if they're being actively used by Mindflayers or the Absolute, or if, like us, their tadpoles have just been postponed somehow and will still transform them eventually. I don't really think that it makes sense for most characters to fully disregard the threat of the tadpoles until they have some concrete answers as to what's going on with it. And the cult very specifically lack those answers. We do know that their mark allows access to extra powers, but we don't know why and we get no opportunities to try and figure it out beyond "it's the goddess." What makes it worse for me is that the circumstances of our infiltration make it to where we can't actually ask questions that would give us all this information. There should at least be journals and bits of text we can find that give us more insight. The fact that they're in the dark about being tadpoled means that it's even more likely they're being manipulated by some mysterious entity you know nothing about. If they did know they were tadpoled then that at least puts them in a position of potentially having more information than you, thus making them a somewhat reliable source of information. But as it is now, the information they actually have is shrouded behind superstition you have to work to uncover alongside all the other work you need to do. Infiltrating a cult is long term work, probably a project involving months spent ingratiating yourself, figuring out who you can manipulate and how.
Ultimately my issue with the Cult path is that it doesn't feel like a path written in a way that I don't find appealing, as someone who's encountered plenty of evil paths in games which didn't appeal to them, I can understand that. This feels like a badly written path.
