A good shielded cable blocks almost completely the electric field of 50 Hz frequency which is created around every electric cable inserted in a active socket, even when the devices are turned off and even when the cable is disconnected from the device.
A shielded cable reduces partially also the magnetic field created by the electric currents in the cable, which is quite small in common cases since the currents are not very high.
Then the bigger effect of the shielded cable is to remove the alternate electric field, which is the most harmful in this case.
The metal shield in the cable is already connected to the ground wire where needed, so you don't have to attach anything else, just be sure that the grounding system of you electric network is OK.
First there is nothing like a fluid in electromagnetic fields, the model of the stream of water in a tube is completely wrong, sorry.
The metal shield around the conductors reduces greatly the electric field outside the cable. Then in the device connected to the cable the alternate voltage at 220 V (or 120 V) is converted into a direct voltage of 12 V or less, in the interior of an iron chassis that shields partially both electric and magnetic fields.
The shield around the electric wires alters the electric field in such a way that the electric field outside the cable is almost zeroed. For this reason cable shielding is intensively used in professional applications, and in high power audio systems, where alternate electric fields would cause unbearable background noise.
The metal plate cannot shield enough the low frequency electric field, it has to make a closed structure around the whole length of the cable.
Furthermore if you build such a metal box it would be rather heavy and difficult to move, while a cable with inner shield is still flexible and movable. Once you have the low rectangular box under the table, what do you do if you need to move the cables in another part of the room ?
Finally, the metal box would shield only the part of the cables that are laying on the floor, how can you shield the part that hang from the tables ?
The backlight of the modified LCD monitors is almost perfectly uniform, some very light non-uniformity can be seen only at few millimeters from the top and bottom borders where the LEDs are installed, and it's definitely not a problem.
The LED strips that we have designed could in theory provide enough light for LCD panels of 40" Widescreen and more, but we haven't tested so far in practice. However there should be no problem at all for 24" Widescreen displays.
Shielding a laptop is not possible in practice because today's laptops are extremely compact and there isn't any space available inside for the shielding, maybe there is just a thin space for a layer of aluminum foil, but anyway it would be too little to achieve a good result. Maybe some shielding foil can be applied on the exterior, but the keyboard has to remain uncovered, so the user's hands are still reached by the EM fields, and there must be many openings for the various connectors, and the ventilation, so the result is not good at all.
If you use the laptop on a desk, then you can move it away and attach an external keyboard to it, and maybe this keyboard can be shielded, or maybe it already has low electromagnetic emissions.