FWIW, I didn't get the LED shutoff button implemented into the Arduino sketch. I went with a safe shutdown script on GPIO-connected button press for the Pi shutdown.
As for the LCD usage (like the example in the referenced Instructables above,) I beat my head against the wall for a bit not getting the sketch to honor the "check current input and display corresponding text on lcd" code where I felt it should go.
I ended up tucking the call to that function into the "if (transmissionSuccess)" condition in the setup loop. Not ideal, but if there is signal being sent to the LEDs, then it checks the current source, and this happens in a loop. For all intents and purposes, it does what it's supposed to.
Here is the sketch I am currently using.
If I get froggy down the road I will revisit this and see if I can't make that a little cleaner.