You're right that the label "certified organic" is somewhat arbitrary. But it's a moot point for most sellers at organic farmers markets because they usually can't afford certification anyway. And many of them do not use pesticides of any kind, and if they do use something like Neem oil, they will tell you.
This does not solve the systemic issue, but knowing the person who knows your food definitely seems like it would be better than anonymous food at a grocery.
The human element can serve to add trust in the place of verification, but shouldn't take its place.
People lie. Industry "standards" change commonly accepted definitions. People buy what's cheap, and effective even if it may not exactly be what's on the tin (e.g. farmer buys an 'organic' fertilizer which isn't, thus 'contaminating' the organic nature of the goods going to your table).
That said, I don't see why higher yield with lower nutrition goods are bad in general. It seems malnutrition is from people not eating enough, not from being stuffed with poorly nourishing food.
If the farmers at local markets are lying about using conventional methods but still growing smaller, tastier fruits and veggies with shorter shelf lives despite still having dirt and rocks on them... then we're totally fucked anyway.
This does not solve the systemic issue, but knowing the person who knows your food definitely seems like it would be better than anonymous food at a grocery.