I second whatever Aniruddh said about the writer being editor. Lot of unconscious/conscious biases and blind spots can hamper the editing process and quality of content if writer themselves edit their work.
w.r.t to payroll or contract, i have seen both work, I have personally preferred inhouse and on payroll always. som points to consider:
1. If you plan to scale the process and onboard a team of writers, better to keep editors also on payroll and inhouse. their relationship can make or break the process/goals.
2. if there is a predictable/repeatable content calendar which you can give good headway for and handover dates can plan well in advance, on-contract editors work well as well
3. if on contract, do not compromise on onboarding to culture/communication principles and product; don’t stop at onboarding only on content guidelines/brand guidelines.