Anchie I like the dresses you have selected and agree that given your air-conditioning at work, you will need toppers or some sort of cover up. I tend to wear cropped jackets or capes in summer with my dresses. My cropped toppers are in finds.
Whilst sheath dresses are often harder to fit, but when you get them right, they are such a simple work dress solution and can be paired with jackets of different lengths. I enjoy midi dresses too, but find that with extra volume in the skirt, the jacket needs to be very cropped so that my waist is still defined - jackets 1 and 2 are best for this.