Reading your last reply to me, I find it very hard to make out what you are actually going on about, especially since a few points are simply untrue.
All the three characters who harbour secrets (Astarion, Gale and Shadowheart) are roughly treated the same in the way their secrets are presented. Gale is the odd one out because his secret is a true secret for both the PC and the player, so his reveal happens much more naturally. In Astarion and Shadowheart's cases, it is obvious to the player what they are, while the reveal to the PC is staged through a couple of events - in Astarion's case, him sneaking off, you finding the boar and eventually bite-night. It is not true that you cannot mind-link with Astarion, which you seem to imply. The PC can do this twice, once when the tadpole connects you during your initial meeting and a second time during bite-night when you can check his claim that he is usually feeding from animals. This second instance is functionally the same as what you do with Gale, invading his privacy to fish for informations.
People have given you several examples for when you can challenge Astarion's believes too. You even brought up your own because I thought the scene when his siblings visit the camp give me ample opportunity to act against his wishes and call him out for being uncaring towards the other spawn. To add another example, if you do go down the "you think power lets you do anything" route in the bear-dinner scene, you can do nothing but challenge his believes. There is no option to agree with him.
I think the only scene in which I truly feel like I am missing the opportunity to challenge him (besides what I already mentioned) is the scene after defending him from Araj on the romanced route. Here he happily tells you that he exploited you and mocks you for it, right after thanking you for not doing the same to him and - unless you want to break-up with him - you can only continue by telling him that you care about him. But I have ranted about this at length in other threads, so I'll leave it at this.
As for Karlach, she was a bodyguard to a rising arms-dealer and crime lord, yet she plays the "I thought everything was about board" card, which means she is either somewhat stupid or deliberately looked the other way while knowing full and well what was going on. So it bugs me a lot that I cannot challenge this story of performative innocence, since I frankly don't buy it.
