I'm not sure what you mean by "natural" for the background. If you mean keeping approximately the original hue, I would mask out the subjects and fill the background with a solid color (new layer, of course), then reproduce shadows using the mask on a another layer and add a darker gradient at the bottom.
Given the warm tones of the clothing, I would go for a cooler background hue:
As for fixing the bottom, do you mean adding ankles and feet?? I wouldn't try that. If you absolutely must have a deeper image (for editorial reasons), you might try a cheap trick like adding smoke or fog at the bottom ...