Re Cirie being too smart to use something without making sure it was authentic.
Seems she knew it was authentic vs. a fake, she just didn't read it closely enough to pick up on the fine print / nontransferable part. I'm guessing it was too late to ask a producer or someone for clarification. To me it was an honest mistake and/or Sarah outright outsmarted her. Here's what Cirie herself said on her Twitter:

cirie fields
@Ciriesurvivor12
The look you have when your mom plays an advantage she didn't read first😂😂 pic.twitter.com/KIWCESzMCG
6:36 PM - 17 May 2017

Re do I believe the party line about this last TC: Not entirely but I do think Jeff and CBS would have loved to see Cirie, a huge fan favorite, pull off that move and finally win. In retrospect, I think they were figuring Cirie would have been the one not chosen on the past swim challenge and were hoping she would have gotten the advantage that was hidden while sitting out. I also think they would have loved to see Brad Culpepper or Andrea or Sandra or Malcolm or JT win and it doesn't appear that's going to happen. So if Sarah indeed wins, as the edit seems to point to, I don't think it was fixed or heavily influenced per se because Jeff/CBS would never want a woman who gives boring confessionals to be the winner of the season. Oh wait, that did happen with Michelle and Sophie, ha, and we saw how they were edited.

I guess I'm not jumping on the conspiracy wagon because even though I agree the edit/TC didn't explain things well this last episode, Michaela made sense from a game logic perspective. Other than Brad, Michaela is the biggest challenge threat and it breaks up the Cirie/Michaela pair.