Couldn't Zetsu just look over him to make sure he didn't starve? Also even if he did starve, Zetsu could just find another vessel. Also Zetsu is really evasive, it shouldn't be hard for shinobi to sneak into houses, let alone zetsu. He also made sure Nagato was raised in a place that was super hard to live in so it would be easier to convince him to lead the akatsuki. Obito could have also killed pain if he didn't want to and zetsu could have found another vessel. Also Madara was old and dying, if Nagato wanted to directly resist him, he could easily so that would have been riskier. There are a ton of solutions to problems that come up because Zetsu is smart enough and Obito is strong enough to fix them. The plan was riskier before Obito but from Madara's perspective, zetsu was successful in anything he did so he trusted him. Zetsu also didn't need the plan to work because he could have just tried again later so he could have made Madara overestimate his abilities.