Az can be really tunnel visioned, the goal of the exam is to do something good for the demon world, and Mephisto wants that good to be creating a place where the oppressed many ears can thrive, and eventually gain dignity and pride from. He seems to have just focused on make "Iruma the king" part and disregarded the whole rationale for the scheme.
If he thought for a minute about it, he'd realise that making Iruma the ruler of the many ears contradicts the purpose of the exam, considering he's never gonna stay longer than a few months and leave a massive power vacuum the moment he goes back to being a school student something which can easily doom a fledgling settlement with no established institutions. The only sustainable option in the long term here is to make the many ears create their own government, that is play the kingmaker.
Also I like how Iruma approached this differently by realising that him being the king would just make him another strong master/patron of the many ears whom they're submissive towards out of fear and a desire for protection, thereby preventing them from actually forming the consciousness to assert their own independence.