Since I assume he doesn't want to have existential risk increase, a credible threat is all that's necessary.
Perhaps you weren't aware, but Eliezer has stated that it's rational to not respond to threats of blackmail. See this comment.
(EDIT: I deleted the rest of this comment since it's redundant given what you've written elsewhere in this thread.)
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.
July Part 1