Should I leave out a degree on my resume?

[Nikia]

2. I still think that a PhD would be more of a plus than a minus

Even if it is in an unrelated field. It shows that you have determination, intelligence, writing skills, and are able to work hard on an original project.
For a Master's degree level job, I don't think that a PhD is that "overqualified". It is only one level of education up. Lots of people with Bachelor's degrees get jobs only requiring a high school diploma and lots of people with Master's degrees get jobs only requiring Bachelor's degrees. Sometimes it even helped them get a job over someone without their degree.