Rereading this, I think this was a happy ending for Rika, too. She's been unable to move on until the end. She knows she's sending Big Imari to Little Imari, and she's promising not to tell lies again. I doubt she physically abused her Imari, but it's also clear she's the one who got Big Imari fired as an OL, and probably partly by lying.
Yeah. Certainly a bittersweet conclusion for her, but I think certainly the healthiest. It's clear at the end that she realized there was something wrong with her behavior and that she was bad for Big Imari, and did something about it. It doesn't make up for the rest, but it shows she wants to change, at least. I think she made a fantastic villain for this story, but the small bit of humanization at the end was an interesting and also welcome choice.