?

I think he saw through Baal's scheme and he wants to bow to the next king. He might even become the left hand of iruma-kun.

I think so too. We've seen a lot of the newer generation lean towards Baal the majority of the previous chapters. Apart from the students, Bachiko and the teachers, Iruma didn't really have an ally outside that circle so it's refreshing to see Mephisto tell him that and I hope he really is.

plus he's hotლ(´ڡ`ლ)

Hot and knows how to be responsible

Yeah probably, also, A reason why i thought Mephisto knew Iruma's a human is because back in the recent chapter a few higher ups know humans exist, Mephisto is a higher up

Indeed. However, almost everyone was in the dark due to grandpa's perfume (the opera showers iruma in the very beginning). Take in account that Amelie was only able to deduce he was human because of the forbidden text she had (her dad too because of his job). Baal and minions only knew because of what the spider dude do during the harvest festival.

If baal notify Callego's brother and brought him to his side is only natural the dude is using the word "human" and grandpa's name as codes for: "I want to be name king and do as I see fit. Only the finest will survive." Because this is one of the phrases the brother says during the 13 crowns ceremony.