Why didn't she just work more so that she and her kids could leave her husband? Because he's not a strong, independent woman trapped in a toxic marriage. She's a needy, manipulative bitch that realized she didn't have it as good as other wives, so she used her looks to take the easy way out and seduce a new man rather than work through her and her husband's issues or just leave by choice, she wanted to leave with a man to blame it on.
First, in Japan it’s not easy to quit a job and find another one. You can see at the beginning of the story Junpei thought his life would be ruined if he had to quit his job. Much harder will it be for female workers. And it’s still many women’ mindset that the way to achieve a better life is to find a better man to rely on. Azu is better example as a more independent woman. But the lamentable thing is, as she’s strong and independent, though she’s very beautiful and considerate, her husband found less traditional female attributes on her (no seeing a as a woman) and ended up cheating on her with a apparently more feminine woman...
How is Rina so conniving but she couldn't just get a job and save up to leave her husband? Why does she NEED to be with Junpei to leave? She had a job and she had time to cheat, take care of her kids, and deal with abusive husband. So she definitely has time to work more. Why won't this heifer just find a new job and work?