I still don't see how Monet is not a Devil Fruit user. She evidently transformed back to human form in the middle of CC's and Law's current conversation. There was no flashback involving Law meeting her and transforming her.
Note that she can flip pages with her wings, but she needs to write in human form, and asking Law to replace wings for arms mid-conversation is rude and disruptive.
Besides, why turn arms into wings when you have 2 working arms and wings can be added on without taking away arms?
It's far more plausible that Monet is a Tori Tori no Mi user, like Pell and Laffitte.
I think you should pay attention to that conversation again. Monet mentions the disabilitated prisoners, we see Law's conditions to stay at Punk Hazard and during the whole present conversation CC was pointing a gun at Law, but in that scene you speak about, there's no gun in sight.
Also, she writes with her wings in a panel of chapter 664. Her arms were actually replaced by wings.
Conclusion, that's a flashback. Monet was a normal girl back then, but now she's not.