I can see the danger this test has for other demons. To be made king of suck weak-willed and broken people would be intoxicating to their demon mindset of strength and power. Even benign ones like some of the Misfits. So becoming the feared king with these people under your command would make realizing, "Oh, I need to raise up one of them to lead them" all the harder.
Iruma's kindness and empathy has him stumble into the answer by being himself.
And for those who think Nova's not ready, note that this process Mephesto is enacting is the same idea as the Three Greats convinced the Crowns on who the next Demon King should be. The next Demon King/Ear King shouldn't be one of the Crowns/Iruma, but someone younger the Crowns/Iruma can seetheir potential for leadership in and molds it to the proper traits.
So much as Iruma is being molded to become the High King of all the Netherworld, Nova is his protoge, molded and raised to lead his own people.