Here's something though, if all he wanted was to give Felice a happy life, all he needed to do is say "Returner? What's that?" and go home instead of choosing to go to hero school. Far from a normal life, what he's giving her is a life as the pet of the school's punching bag. And let's go over some other flaws there
1) He's worried they might injure Feline to get to him if he goes all out? Screw that, he's putting Feline in danger because because he's trying to act like some random schmuck off the street while attending a school filled with "heroes" that are just incarnations of raw violence and hatred.
2) He and Feline already effectively proved that the goddesses' seals don't mean much more than tissue paper. Feline summoned him, presumably without breaking the seal, and then they returned to Kyouya's world, again, presumably without breaking the seal.
2a)If the two did break the seal at some point, and the goddesses have any way of connecting either of them to that world, then they'd either already know because they were alerted as soon as the seal broke, or figure it out very soon regardless when they check on the seal.
2b)if the two did not break the seal, it means the goddesses' seal is at most nothing more than a mild inconvenience to the two of them
They showed up right when he got back, gave him his summons, told him showing up was not optional, and immediately left. He wouldn't have been able to avoid some sort of interaction. Even if he claimed not to be, they still would have shown up and tried to make him attend, and that would have put more of a focus on him. Also, he's not the school's punching bag. He has shown himself able to avoid being harmed from anything he has faced, and just hasn't escalated any conflict. He has actually protected others from being punching bags, and has stood up for himself if violence was threatened.
To address the "flaws" you suggest.
1) He's not trying to act like a "schmuck off the street", just a "middle of the road hero". (You know, there heroes that are all stated to be able to potentially destroy worlds. I don't know many schmucks off the street that can do that.) Any time that somebody has threatened Felice, he has protected her. He doesn't see the heroes as enemies, automatically, and any heroes that try to become enemies, he is willing to deal with, so far. Not picking fights or declaring how "alpha" he is should not be seen as a show of weakness. In fact, the guy that tried to assert how "alpha" he was got shown to be a weakling who was trying to cover for his failure, whereas Kyouya knows his strength enough that he doesn't feel the need to prove it to others, or to pick fights, even if he could win them.
Not to mention, Felice seems to like the idea of him attending the heroes' school before they start their new life. "Kuku, it's fine, isn't it more thrilling like this?"
2) We don't know how long Felice was trapped there. In general, when "seals" are used, the longer time goes on, the weaker they become. Kyouya and Felice also don't know if they've been improving their sealing ablities, in case Felice does break out. But if they can be sealed for any amount of time, it still gets in the way of their peaceful life on earth, even if they can overcome it.
2a) The world is called a "Lost World" which might be because they lost to the Demon Lord, or because after it was sealed, they lost track of it. Even if they know where it is, if they aren't monitoring that world all the time (because they are busy monitoring a ton of other worlds that are not sealed and have active Demon Lords), they might not notice if the seal is pierced, but not shattered. If enough time has gone on, they might not check it as often, and not realized it was broken. If they haven't noticed, then Kyouya and Felice don't want to bring attention to themselves and have them end up noticing. Even if they have noticed, if Kyouya and Felice don't know they have noticed, they would still want to keep a low profile. If they have noticed she got out, and Kyouya and Felice know they have noticed, then they definitely want to keep a low profile, unless they know that the goddesses were able to trace Kyouya and Felice's return. Because if all the goddesses know is that she broke containment, but don't know what happened to her, they probably would expect her either to come at them for revenge, or stay far away from them, so being at the school, but not seeking revenge helps her avoid notice.
2b) It might be a "mild inconvenience" that takes time to escape from, that would interfere with their time on earth developing relationships with other people, and enjoying all of the Japanese cuisine that people in other worlds long to eat.
Even still, there were still plenty of reasons I listed, beyond even these, that give reason enough for them to not seek out conflict. So if you are still missing something, it is because you are trying to not see it, rather than that it isn't easily visible.
Also of note, Kyouya and Felice talk about the idea you have, of asserting his power (with Felice suggesting he might be able to do what you he should). She says: "With your power, which surpasses even mine, I want to know what you are going to do here at this academy. Should it be you, you could rule this school, conquer the world, or do anything you want."
He replies to her (and you): "Even if I could, I'm not interested. The consequences of wielding this much power... You've seen it firsthand, right? Besides! If I stand out, they might find you out. I don't want that. I'll just live my life as peacefully as I can. As for fighting... I've done enough for the rest of my life."
So, yeah, she says he could rule it, he says maybe so, but I don't want a life of fighting. (Since he has fought for tens of thousands of years, that is understandable.) He also knows that, if he were to wield that much power, it would be very easy for the world to get destroyed in his fight against the goddesses, and that would ruin his plans for a peaceful life on earth with Felice.
P.S. I think your autocorrect kept changing "Felice" to "Feline".