But if you recognize the right of self-determination for yahoos that want child brides, that is negating the self-determination of the children that are victims.

Do children have a universal right to self determination? We protect children from abusive and neglectful parents but we do also recognize that good parents have some authority of their children. We don't let children run away and join the circus or eat nothing but candy till they have diabetes.

What would be the problem with a patchwork of micro-nations?

Cuz it tends to create ethnic conflicts and war. Yah know like in Yugoslavia. That's literally where the term balkanization comes from. Plus few places are totally ethnically homogenous, what if my home falls in the border of one micro nation despite me preferring the government of the micro nation just to my south? What if my village is split 50-50 between two ethnic groups, which micro nation do we join?

These nations could form federations or whatever to complete projects that may take the resources of a large nation, or whatever reason there would be to want large nations versus micro-nations.

This is how most democratic nations already work though, they divide their territory up into regions that have some local autonomy in political matters.