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?
Oh, I can't imagine that it didn't cross her mind. Do YOU never experience ambivalent feelings? Why should she be any different? I've already said that I think she behaved VERY badly. In fact, she behaved like the trollop many were saying she was. Who of us haven't done things, in our younger, sillier days, which come back to haunt us now? Perhaps we feel more shame about them now, than we did at the time? I imagine she's worked very hard at leaving that person behind.