The plan wasn't a failure. The current results are that most of the stronger students are more incline to side with them while the weaker ones realize the situation and are scared to get involved, having lost moral. Give it another chapter or two for them to be more obvious but we can already see some results. Their opposition is now fragmented. Also, they knew about the slaves, that's how they knew to spread the rumors of them among the heroes.
As for the mc reactions, I think it's pretty reasonable, even if it isn't exciting. People tend to have more calm or muted reactions to bad situations if they have someone giving them orders. Even without that, many more people than you would expect have emotionally muted or delayed reactions to death. It's actually a problem where people with more open reactions start getting mad and demanding the more muted people to act like they do. Therapists have to constantly tell people that people process differently and to not ridicule them for it.
That said, the author probably intended for it to be chunni but the reaction is still more common then you'd expect so no point criticizing the way the mc is acting.
