I really don't get why they didn't just recast Kang. Surely Jonathan Majors isn't the only actor who could have played the character. They changed War Machine and Hulk with no issues.

Cyborg was put into the Justice League because he was a contemporary of the Titans who had grown up and "graduated" from Teen Titanhood - primarily Wally West, but also Nightwing, Arsenal and Tempest, so it made sense that he was ready to the adult hero team. Young Justice was ready to be rebranded as the Titans, but they were the generation younger than Cyborg.

The imperial Japanese of World War II were America's enemies and were cruel to their conquered subjects and allied with Hitler. A comic book of the era was well within morality to call for harm to be done to them. They weren't being demonized for their race, but for their actions.

That's a commonly repeated myth, including once in a Silver Age Superboy story. But if you check the entry for Cinderella on Wikipedia, it pretty much says that that's not true.