I think her wish was the inadvertant "I don't want to die." That's where the immortality came from, but the thing she summoned took the villagers' lives in exchange.
Miwa's trying to get past her bullshit and grow as a person. Saeko, up to this point, doesn't seem have any interest in growth. She's just less mature than Miwa is, funnily enough. More experienced, but less mature.
Hope you enjoy your studies! Don't forget to take breaks and relax at times. Appreciate your work on this series. I've enjoyed reading your translations!