Re: complex comma situations
I would use a dash instead of the first comma. That would indicate that the rest of the sentence describes the tableau. The comma after horsewhip separates two phrases. Thus, it would be: "We had long thought of them as a tableau -- Miss Emily a slender figure in white in the background, her father a spraddled silhoutte in the foreground, his back to her and clutching a horsewhip, the two of them framed by the back-flung front door."
Originally Posted by mas94010
If a sentence lacks clarity, rewrite it until it is understandable. There is no excuse for anybody not to follow that rule--especially writers.