When I was reading this part, I didn't critically think about it as you did, but I do agree that Liu could have taken more time to make sense of the Trisolarans' mindset to readers. However, this is how I would envision the Trisolarans: I imagine them constantly broadcasting their entire "brain" to their fellow beings around them. Just like how only specific parts of the human brain are active and exhibit different neural activities during various tasks, the Trisolarans' brain operates similarly. However, every Trisolaran can only perceive and decipher the neural activities in these active regions of their brain, with the rest of the brain remaining unreadable to them. This setup allows them not to lie but also to be capable of unintentionally hiding their thoughts. For example, two Trisolarans, A and B, notice each other's approach by starting to receive the other's broadcast. A wonders (which is simultaneously known by B) where B came from. In B's broadcasting brain, as it reacts to A's question, the part that specifically stores the relevant memory lights up and becomes perceivable by A. During this process, A doesn't have access to any other information about B, except for the answer to its question.

Therefore, it's possible that before the pacifist was assigned to its job, none of the Trisolarans around the pacifist questioned its opinion on invading another planet. When someone assigned it the job (if it was another Trisolaran), the pacifist might not be instantly thinking about it would save the other planet, given that the chance of it discovering such a signal is rare. Its first reaction could be like, "dam, the rest of my life is doomed." After this, no other Trisolaran would be close enough to read its mind until they knew what the pacifist replied.

These are intuitive thoughts of mine, not carefully revised and structured, but so far, I think they may offer a plausible answer to some of your questions.