From memory the first time that ricin came into the show Walt and Jesse made it from some beans on Jesse's bench top with a pretty crude MacGyver type setup. It didn't seem to require a complex lab or equipment.
Grabbing some ricin to off someone isn't like grabbing a gun and blasting. He'd have to have thought out how to use it and if he had time do that he'd surly have time to make some more rather than risking going back to the house to collect it. Bird next door who saw him is going to call the cops, that shits going on TV and aside from the obvious problem everyone is going to be on guard when Walt offers them a burrito.
Think this bit could be to mislead or something else altogether.
Also, don't get why Saul had to run.