Okay, I've determined my new button configuration and the matrix setup. It will be a 10-pin matrix, with the 17 button number pad and 8 additional function buttons. Here's the sketch with each button labelled with its respective position in the matrix array. Pins will be labelled A-E and 1-5, with 25 possible combinations, one assigned to each key.
Also seen in this picture is my planned layout for the 8 function buttons, complete with LEDs and "glowing traces" on their panel. These will either be EL wire or painted, and if I can figure out how I might use recessed NeoPixels instead. As with most of this project, we'll just have to see what works out.