ok so
1. I don't know what you are trying to say
2. I don't see that as relevant. Israel is fully of a people who have been oppressed for two thousand years and there are cultural reasons for them to be insular.
3. It depends on which
Welsh or
Scottish person you talk to. In neither case does even 50% of the population support independence.
4. You may be correct about the Balkans, that is a pretty fractious region of the world and even in nominally cohesive nations, they seem to find a reason to fight amongst themselves. Perhaps that is just slavic culture since we don't see it so much in other regions of the country.
5. In this case I blame the fundamentalist Muslims and their belief that they should be the dominant religion of the world
6. Administratively created countries are a problem, we see that in the middle east as well.
The problem with what you are presenting is that there are a myriad of causes that can be traced to things other than ethnic diversity, whereas there are examples of ethnic diversity working. In that, I present England (I found your evidence poor, so I am going to use it as a positive example), the Flemish and Dutch, mainland Italians and Sicilians, the Creole and English speakers in Louisiana, Hispanic culture within Texas, the general hodge-podge of peoples in Taiwan.
There have been any number of example of stable and prosperous multi-ethnic countries throughout history. I just listed some examples.