https://www.gregmat.com/quizzes/quiz/gre-mini-exam-2-may-13-2022 (Question 1since they conspired, still they did not feign it…
“…harbor feigned antipathy..” is not a natural expression. However, if we look beyond that, “even though” sets up a contrast between the idea before the comma, and the idea after the comma.
Using “feigned” in B2 doesn’t establish this contrast. If anything, we would expect people conspiring to overthrow a government to harbor genuine rather than feigned antipathy toward it.