First you need to realize that AWS, Advance Wireless System, with Nikon's CLS Speedlights and cameras use IR, Infrared, light communication between the Master/Commander and Remotes. This light can be reflected or blocked. Reflected IR is not always a problem but blocking of the IR light will prevent control of the remote units.
Using a box or umbrella is more subject to blockage of light form your camera's pop-up light. Only experimentation will determine if this is a problem.
To change the power on remote Speedlights one can only change the power level of the group of Speedlights. To control 2 different Speedlights power level requires each Speedlight to be in a different Group. You then set the power for the Group through the camera's Flash control menu using the "Comp" field.