Of course, less qualified guardians of an individual's self interest who believe themselves to be more qualified are a legitimate risk, but that doesn't mean that the optimal solution is to have individuals act exclusively as guardians of their own self interest.

The optimal solution isn't necessarily to have someone else act as the "guardian" of their self-interest, however well informed. BTW, I don't know if this is true of American English, but in British English, one meaning of "guardian" is what an orphan has in place of parents. Doing things for adults without consulting them is usually a bad idea.

If the Kenyan Ministry of agriculture follows the prescriptions of the researchers in the article cited above, they will thereby become better guardians of those farmers' interests than the farmers have thus far been, within that domain.

Not if they try to do so by simply coming in and telling the farmers what to do, nor by deciding the farmers' economic calculations are wrong and manipulating subsidies to get them to do differently. (I've only glanced at the article; I don't know if this is what they did.) Even when they're right about what the farmers should be doing, they will go wrong if they use the wrong means to get that to happen. Providing information might be a better way to go. The presumption that if only you know enough, you can direct other people's lives for them is pretty much always wrong.