A proposal
Stop asking people who are new to this to police gender expression
Have at least one staff member go through some quality diversity training. If gender must be policed, let that staff member do it.
Follow this excellent advice. Consider the risk of treatingbeing nice to a hypothetical troll who performs a non binary gender expression nicely, as if there is nothing at all odd about it. In fact, allowing skeptical people who are unfamiliar with nonnormative expression to try it out is an extremely useful exercise. Hypothetical trolls may feel free to walk a mile in my shoes.
Generally, dial the troll detector way back when it comes to user profiles and pronouns. If it's not explicitly offensive, leave it be.
Counsel moderators to not, ever, call someone's pronouns made up. If they were flagged and someone with diversity training believes them to be offensive, say something like a user found your pronouns offensive. The phrase "made up" does not read as "something you developed on your own to express your individual identity". It contains within it the idea that the individual's identity is concocted; falsely fabricated or invented. Even, e.g., a sexually explicit pronoun may be valid. It is just crosses the line for this site. Don't put yourself in a position to guess at someone's intentions. State the facts, without judgement.