yes after it was revealed that he is the reincarnation of lucifer the fallen angel, which explain how can he wield the power of light and dark, the duke and the emperor were banished to the shadow realm

The novel - obviously.

did not her memory was erased thus how she was made a maid, later tho she recovered those memories

i think the king title changes every tournament or so

It's not weird to see him become "a king" if it's purely an honorary title. You go to the tournament, you win, you become "the king". Cool. You get a neat-looking crown, then you go about your day. That's how casually it was presented, too, and the mannerisms of the other characters also imply that it's more of an honorary thing than a legitimate thing.

So let me ask you this. How in the hell would any functional governing body work when their primary form of getting a king is inviting every foreign nation to fight over the reins? And then I get to listen to one of the main knights say, and I quote, "Making sure that Reinhardt is a complete and independent nation... wasn't that our dream?" Am I supposed to be taking this seriously? I don't see any justification to these developments at all, I don't see any justification for why this whole tournament form of executive power should ever exist with this kind of "dream", except as a flimsy excuse for Joshua to get political and military power that isn't beholden to the emperor. Right at the time he's told the emperor is doing shady stuff and he assumes a continental war might start, I might add.

Right now, the story has done a poor job convincing me Reinhardt and its knights can logically exist in the world as anything more than a writer's convenience to push some contrived plot developments later.

I would useless rather than wasted. If he was a more prominent character I would say it was wasted, but even his powered up form looked generic. Absolutely wild.

It's shown up a few times over the years. I've seen it in Bleach and some other minor mangas.

I will support the man who kill babayaga