Okay, so the blue image is the bodywork regions allowed in the rules, it's from an older draft of the rules so there's no loophole for the T wing, and red image is where bodywork cannot be sharper than a 75mm radius. The red box sits a little higher than the floor, so the floors edge can be sharp and you also get the strakes and cutouts (tyre squirt slots) ahead of the wheels. The blue shows how if the sidepods extended to the full width of the car then they would have to fit where the box steps down twice ahead of the rear wheels (basically to axle height).
How the double diffuser worked was that there were holes in the vertical face between the step and reference plane, now this loophole is closed, so while the upper deck was above the floor it helped to increase the negative pressure region on the lower surface of the undertray through the holes. If you put a wing, or any downforce generating device, above a closed surface what it will do is infer a negative pressure on the top of that surface, generating lift, where you actually want high pressure on the top of a downforce generating component. I think what that 'wing' on the Williams is doing is helping to accelerate the airflow out of the cooling duct to reduce drag, as well as acting as a flow conditioner to the top surface of the sidepod.