So I managed to miss the disclaimer that said the 12.1" screen would need a 12V 1A power supply. Luckily, I keep all of my extra wall-warts stowed away, just in case something like this happens. The closest three I have found are a 12V 500mA, a 12V 900mA, and a 16V 4A one (not close but the only one more capable)...
I decided to try the first two to no avail. I figured the 12V 900mA one would definitely be close enough to power the screen, but apparently not... Now, I wasn't just going to plug in the crazy 16V 4A one, but I'm starting to think that something else may be wrong. I don't know how the screen is supposed to "turn on" as I've tried all of the buttons on the breakout board for the controls and I don't see ANY LEDs light up when I plug in the barrel of the first two power supplies.
Obviously, I'm going to run out and grab a 12V 1A power supply tomorrow and will try that, but as mentioned, I think something else might be going on considering how forgiving the RPi is with amperage and how I was only 100mA under for the screen.
I also didn't add "hdmi_force_hotplug=1" to my RPi 2 config.txt file because the 12.1" screen is powered externally, not by the Pi, which was the reason given for the instructions for the 8" screen.
Can anyone describe the behavior of a working 12.1" screen? How do you turn it on? Does it need to have a signal to even light up and possibly light the LED in the control board? Anything would help, thank you!