The following quote is from Sister Carrie by Theodore Dreiser:
What Drouet said about the girl's grace, as she tripped out evenings accompanied by her mother, caused Carrie to perceive the nature and value of those little modish ways which women adopt when they would presume to be something.
She looked in the mirror and pursed up her lips, accompanying it with a little toss of the head, as she had seen the railroad treasurer's daughter do. She caught up her skirts with an easy swing, for had not Drouet remarked that in her and several others, and Carrie was naturally imitative. She began to get the hang of those little things which the pretty woman who has vanity invariably adopts.
Would you please explain the use of the subject-verb inversion in "had not Drouet"? and what is the meaning of "had not Drouet remarked ...?
Does "for" mean "for the reason" or "because"?