While everyone may be celebrating that the Marie Marriage route has been unlocked, we do have to grapple with Arthur realizing this.
There are a few routes I can see. The rom-com route, where he tries to get her to admit to being human via dates and stuff, or the gloom route, where he immediately fires her, and then realizes how miserable he is without her around.
Ultimately, I predict that no matter what, Arthur will realize his love for Marie is stronger than any mistrust he has of her lying about her identity to get close to him.