Let me see if I have all of this straight after tonight's episode.
There was never a switch of cigarette packs. There was never need because Walt tricked Jesse into thinking the ricin cig was sucked up by the roomba. The cig has a vial of salt in it because he wasn't going to give it to Jesse. The actual ricin vial is in Walt's bedroom electrical socket. sse has obviously been becoming more wary of trusting Walt, realizing just how often he is manipulated by Walt, and his suspicions from season four (about Huell lifting the ricin cigarette off of him at Saul's office) were only confirmed when he realized that his weed had been lifted from his jacket by Huell as he was leaving. That made it click in his head that he was right the first time around in season four, when he had the gun pointed at Walt's head, assuming he was the one who poisoned Brock.
No, he was not poisoned by the ricin, obviously. But the ricin cigarette missing was the catalyst that set Jesse on a path for revenge, against whoever poisoned Brock, and Walt manipulated him into believing that was Gus. It doesn't matter that it was Lily of the Valley, the missing ricin cigarette is what matters, because it was the tool used to set Jesse off. Jesse knows how smart Walt is, and knows that he is capable of coming up with a plan that complex, using a plant that creates symptoms incredibly similar to the effects of ricin.
Sound about right?