They didn't need to butter her up, and promise immunity. She went to the police and made the accusation, she made statements, they didn't need to promise her anything.
As for this "relief" she must have felt, did she feel anything else? Shame perhaps? Did she feel she could have prevented the death of those five people, including two little children? Did she say sorry to everyone for covering it up, and for supporting Jeremy? She sold her story, so where was her shame then?