The reason your tank is nearly overflowing is because you are restricting the drain by using the 3/4 pipe. You will not have the problems you are seeing if you change out your drain line to 1" pipe.
Your drain line does a 90 turn (last pic in your first post) the horizontal part of that pipe is it level, angled down or angled up slightly. If it is slightly angled up it will restrict the downward flow of your drain line. Remeber that your drain line depends on gravity to work as well, not just the pump pushing the water into the tank and then into the overflow.
If you add another U tube to your current setup you will overflow your overflow box. You need to change out your 3/4 pipe to 1". After you do that add the second U Tube if you want but it will not change the amount of water that can flow down the pipe attached to the bottom of your overflow box.