Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
None
This is a drama, so there's not a narrative point of view going on, per se. That said, looking at how characters and their perspectives and opinions are presented can sometimes give us a clue to the story's overall stance toward its subject matter.
The play alternates between acts in which one side of the British/Yoruban tension dominates; Acts 1 and 3 focus on the ritual Elesin is about to complete and preparations for it, while Acts 2 and 4 primarily track the British response to this "threat" from the Yoruban community. Act 5 brings the two sides together, with Simon and Elesin (each the most important figure in his community, as far as the play is concerned) forced to confront each other and their differences in opinion. So, in this sense, the play's portrayal of the conflict seems pretty balanced.
However, Soyinka definitely portrays what happens to Elesin as a tragedy, which means the play itself at least somewhat aligns itself with Yoruban customs or values. So there you have it: There's no official narrator or single perspective, but Soyinka still lets us know who the good guys are.