] Here's what I found....

] a) There's no place in Normandy that is now, or was, as far back as the sixteenth century, that is spelled Arcy, or anything like it.

When a name refers to a place (starting with 'de'in French, 'van' in Dutch, 'von' in German), this doesn't mean that this place has to be a town or city. Lots of names mean "of the mill", "of the corner", "from downstairs", "of the river", etc.
For example, my very own name Van Besien means "from the side" (indicating my ancestors must have lived in the outskirts of town) and not "of Bésy" as we had previously thought.
So D'Arcy could come from D'Arc or D'Arche. And when they moved to England they may have changed it to Darcy. This is just a guess though.

Also, when a name refers to a city or town, this doesn't mean that the ancestors were landlords. More often than not you'll find that in fact the great-great-...grandfather was a foundling. They were usually named after the place where they were found.