Hi,
I used the outline tool to try to do what you did, however, when finished, I was unable to do anything further. It's really difficult to get a smooth cut anyway.
I tried two different outline plugins. After adding them to the "Effects" folder in Paint.net's programs folder, I'm unable to get them to work.
Additionally, I tried the magic wand tool, however, when using it, it ended up selecting pixels inside of Kimidori Emiri so I can't use it.