After thinking about it all night, I can definitely believe Walt would poison Brock, but I don't know how he would have pulled it off.
In another "breaking bad" moment, Walt is faced with the need to get Jesse back on his side. Walt never would have killed a kid before, but if the alternative is his entire family is killed? Well, if the options are Walt's family dies or he poisons Brock with Ricin (likely even telling himself Jesse will figure it out and the docs will save the boy, making him not a killer at all) I think he'd do it.
Also, when has Walt ever been the one to just sit at home and wait to be killed? He's always been proactive about fixing his problems. Sitting at home and waiting for Gus send wave after wave of killers at him isn't his M.O.
As has been mentioned though, while Saul's guard might have lifted the Cigs off Jesse, we haven't seen how they would have planted the Cigs back on him.
I think the writers intentionally left just enough holes in this because this is a conspiracy they want us to view as a possibility, if only to make us consider "How bad is Walter White?"
Question: Anyone have any idea how long its been since the pool party in Mexico? The Mexican doc said it would take Mike a week to recover. Any chance he's back and staying out of sight yet? If he's back, he could easily be tailing White (or, really, anyone could be tailing white) and telling Gus to stay away from his car, then Gus would go outside just to make a show for Walt and mess with his head by walking away from the car.