Manually Change from one Power Source to Another
Thanks to your drawings and an image I have found elsewhere of the insides of your controlled outlet, I got a very simple solution for you.
Here is the image with my suggestion:


Basically, all you have to do is bring out 2 wires (in a single cable, for safety, reliability and convenience) and connect them to a plain 120V switch (get whichever one you like in a store).
If you don't want to go inside the box and mess with it, you could use two 120V power cords with plugs on one end, strip them on the other end, and connect their live (hot wire, black color) wires to the switch. One plug has to be plugged into the non-switched outlet and the other one in any of the remaining 2.
MAKE SURE THAT BOTH WIRES GOING TO THE SWITCH ARE THE HOT SIDE, SO THAT YOU DON'T MAKE A SHORT FROM HOT TO NEUTRAL!
This second method would leave you with only one, switched, outlet and you would need 2 cables/cords instead of one, but you wouldn't need to go inside the box.