That's a lot of words to be wrong, friend-o.
Saying "Are you sure you don't want to revise your testimony, because we have factual evidence that it's false and that may lead to an indictment for perjury." isn't extortion, it's allowing this guy to not loyalty his way into a felony.
Imagine, the police/prosecution use their discretion and don't throw the book at someone and idiots still take exception to it
Sure you can. Especially if he can bring receipts related to why he was lying. Threats, promises of compensation, etc. Is it ideal that he lied under oath? Of course not. And it opens a giant door for the defense to challenge all of his testimony but it's not irrecoverable.
Much as you seem to wish it was.