I don’t understand the question. The Adafruit libraries make it easy to set the pixels to whatever you like, so for your particular project it’s just a matter of coding it up. I’m not sure I understand what you mean by “cascading”.
Maybe I just misunderstood. Whenever I connected more than one Adafruit matrix panel the images would duplicate (plugged the second panel into the output of the first) instead of having separate images on each panel. I’ll take another look at the code/libraries.
The USB port on the Raspberry Pi is at 5V. When you connect a power source to the backpack using a power adapter, that also sets the 5V line on the FTDI cable to 5V. So both the Pi and the backpack are setting this line at 5V, which is fine. No current flows because they are at the same voltage. If you unplug the Raspberry Pi, then there could be current flowing into the Pi, depending on the design of the USB on the Pi, but it probably has a diode in there somewhere. Having all this connected and powered at the same time is fine, but don’t remove power from either the backpack or the Pi.