There will be more to say with regard to the specifics of the first part of your question, but I addressed the issue of ethnicity on the Swahili coast, especially with regard to the question of whether the elites there had Bantu or Arab/Persian heritage, in an earlier response on the nature of the government of Barawa, a town which forms a Bantu cultural enclave on the Somali coast.

You might like to review that response while you wait for fresh answers to your query:

What was the nature of the government of Barawa? Was it unique in the Swahili coast? Was it a republic, and if so, how did it compare to contemperary governments in Europe?