There is no reason to other than "hurt a MC". She is a good person, even though that was a shitty deed. She had a understandable reason for behaving like that. Also I wonder if you'd hate her so much if she were a man since you are using a sexist slur.
Okay I'm going to pretend that they'd live with the old man too. As a thank you for keeping Emma safe all this time and taking care of her, they decided to make be a family with him hearing that he lost his. Mujika and Sonju is running the demon world now, well mujika is while Sonju must be travelling around keeping things in order. The agreement still stands as Emma's memories of her family is forever lost but the emotions run deep within her heart and soul. Memories can be made again. As long as they exist.
They need to get to the country before the demons arrive there, if they send people to stall for time they'll save more people than have 10 people try to catch 2 Demons. It's not about saving Armed forces heading towards battle, but to catch demons before they run away or kill people. And dont tell me the demons are too powerful that'd be even more bs.