Haha, I don't know if its gone honestly. Sera just has bigger concerns right now.
Nah, I think he probably didn't want to focus too much on the mental magic because it was leaving anyway. But Arisa herself mentions that because her level wasn't that high anyway, her mental magic wasn't even that impressive. The whole reason it even worked on Satou to begin with was because of her Unique Skills. In both cases you are looking at whether to make use of something that is leaving regardless, so ultimately, its inconsequential whether they use it or not to start with. Because you end up in the same place regardless.
I can see that, because like Momo, Arisa pushes the harem because she doesn't have the confidence to steal Satou for herself. That being said, I feel the person who can make Satou a harem protag is just Aze. She is probably the only person who can convince Satou and even then its a hard sell. She tried to convince Satou initially because it was the Creator God's law, but Satou could tell its not what she actually wanted herself. Aze wants to monopolize Satou just as much as any girl, she just doesn't think she has the right too. So honestly, if Aze wants the harem to happen legitimately, and not just because its a law, but rather, she wants the girls to be happy then she could probably convince him. I feel if she makes it clear that she appreciates Satou putting her feelings first and even denying the harem for her, she probably would feel more comfortable with it overall. However, the issue here is that Aze has a submissive attitude, especially after Satou himself because the Creator God.
That is where the real problem lies, because Satou loves Aze, but he is also technically Aze's Lord and Master. So Aze, who is very devout, doesn't really have the mind set to argue against Satou's desires. Especially now that is the Creator God, so he can straight up remove the previous Creator God's propagation law. Meaning that Aze has an even harder uphill battle in convincing him.
The Great Demon Lords are the Demon Lords far stronger than the normal seasonal Demon Lords that appear every 66 years. For example, Golden Boar Lord and Dog-Headed Ancient King are both Great Demon Lords. This much is clear because despite being "Demon Lords" they are level 120+, which is far stronger than anything Hayato can handle. The legend is supposed to be that when the Great Demon Lord Season begins, the Founder Hero of Saga will descend and battle them on behalf of the Gods. The Founder Hero in legend is supposed to be the Subordinate God that sealed the Demon God hundreds of years ago. Despite all the propaganda, that legend is technically true, because the name of the Founder Hero, the first Emperor of Saga, is a name you might recognize, Hero Suzuki Ichirou.
But yeah, like I said, they specifically set up the role to be Vice-Minister instead of Minister because they were already eyeing Satou for the role long before the Ministry was even announced. And long before Satou was even interviewed by Dukus for the role.
Its not that big of a deal, many side characters fall into the background as the characters get stronger and stronger. And to be honest, they just represent "peak humanity" since its very hard to surpass level 60, because that is widely considered to be Hero territory. And the side stories explain a big reason for that is because of the massive EXP requirements to get that high, which is easier for Heroes to get to because they get double EXP from fights.