Breaking Bad Finale Discussion(Spoilers from previous episode, if you haven't seen it don't read)
I think that Walt poisoned Brock with something other than ricin. I think when he was spinning the gun and they landed on the plants. He realized there was another toxin he could use to mimic ricin that was less fatal and faster acting.
The reason that Gus didn't get into the car was he realized what Walt had just done, and he knew that he had turned Jesse.
The cigarette was stolen from him when he went to see Saul.