Aaron,
If you can not calibrate the touch screen, there is a hardware issue with the screen or the connection to the Nano.? Remove and visually inspect all the Nano pins and the traces to the screen connector.? There must be a break or short in the lines to the original display.
The Nextion uses the same wires for the display and the touch screen.? There is no screen calibration for the Nextion.
Checking the schematics, I noted that the Nextion uses the same Nano pins as the touch input for the stock screen.? Since you are having issues with both the Nextion and the stock touchscreen, this would lead me to believe there is a problem with the wiring on the Raduino or that the Nano IO pins have failed.?
73
Evan
AC9TU