In a recent interview, Bryan Cranston confirmed that his character did in fact use the Lily of the Valley to poison Brock knowing it would win Jesse back.
“Walt was desperate. He realized that Gus had turned Jesse into his favor, and it would take something extreme to bring Jesse back into his camp. And he needed Jesse to be able to draw out Gus. Otherwise Gus was going to come after him. You want to be proactive if you’re going to protect yourself to go after him. So, what could it be? “How can I bring Jesse back in? What can I do to prevent Jesse from cooking that would bring Gus out into the open?” And he thought: A-ha. And it had to be something I could turn and blame Gus with.”
To read the rest of the interview, check out TVLine.com