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.

What's this rubbish mean? Don't you realize that Ielts school firstly sends its client schools the teachers' resumes? Secondy, the schools accept or reject said teachers on that basis.
Another point, how do you know the Africans are barely understandable in their work environment, as opposed to native speakers of English? Probably the client schools that accept them do so in the knowledge they are getting a good deal because they're shelling out less to Ielts School, which will mean the Africans are earning less, but the students' English is so pathetic there, so they couldn't appreciate any kind of pronunciation. Incidentally, know you not there are native speakers bred in Tyneside, for example. But nobody except their fellow Geordies can follow what they are saying! Likely they are more difficult to follow than Africans. As an FT, you are so naive that it's unbelievable! [edited]