I do understand that all of EV's teachers are native-born English speakers; whereas, ielts relies on barely understandable Africans for the bulk of its workforce.

I don't know what you say about EV is the truth, but I know for sure what you say about IELTS school is misinformation. Actually, the bulk of its foreign staff are white folks these days. You should also be informed that IELTS school will only employ a foreigner to teach English if a client school has agreed to accept him (or her) in the first place. How understandable the spoken English of the foreign teachers is isn't and shouldn't be the issue here as they aren't teaching you nor me. The client schools have accepted them. So, IELTS school gets paid. This means the foreign teachers get paid by IELTS school.