I do agree that just simply joining the cult and going along with what it wants surely can't seem a good idea to anyone. But given that the PC is going to have to deal with the cult in some way anyway as they're clearly associated with the tadpole implanters, and given Halsin's ability to help is doubtful at best, a character might decide that it's worth a punt to get in with this cult, hopefully find the higher ups who know what's going on with the tadpole or who can lead you to someone who does, enabling you deal with that problem while - you never know - possibly getting a position of power and influence within the group to use and subvert it to their advantage.
I do agree it needs work, though, to seem compelling as an infiltration operation for the purposes of either destruction or takeover. Though I don't think there's any need to have different starting points in order to do that. Just having one of the companions compellingly argue in favour of infiltration (I vote Astarion) and changing dialogue options when going along with the cult so that we can register our intentions, and gain confidence that the game recognises it as a valid motivation (and therefore likely has the content to allow us follow through) would go a long way for me.
And you could make some relatively small changes such as not having the goblin camp turn hostile if you side with Minthara and get rid of any dialogue that explicitly rules out infiltration as an option. Other than that, I'm not quite sure what you mean when you say that the infiltrate the cult option fails immediately after the grove, though? I confess I've only gone that route once and I don't recall all the details.
And I actually don't think it matters that there are ways to infiltrate the cult without helping Minthara, as long as we have a choice to roleplay how we think our character would approach the task. In fact I think it's positive if there is overlap and commonalities as it means that the different paths can possibly converge and diverge again, with different flavours and options depending on what you've done before, but leveraging a lot of the same content so saving development resources for Larian while increasing the flexibility for us to play twisty or conflicted characters whose allegiances might shift over the course of the game. I would be really disappointed, on the other hand, if siding with Minthara locked you into a wholly different path so early in the game.
What was said in the last Panel from Hell about the choices becoming more complex and grey as the game goes on gives me some hope that this is what Larian are going to aim for, though admittedly it's no small task to pull off.
