Assign multiple outputs to individual inputs via shift registers

Other Hardware LEDs and Multiplexing
2

Work out what bit position changed don't know how to do this

Use an XOR

byte changedBits = oldBits ^ newBits;

For each bit that has changed, changedBits will contain a 1; non-changed bits will be 0.

Further, if you change

output_t outputs[5]; //can have max 5 outputs for each button; this holds an array of output_t objects

in your button_t struct to

output_t *outputs; // outputs
byte numOutputs; // number of outputs

You do not have the limitation of 5 outputs; due to the use of a byte variable it's now limited to 256 (and if you use an unsigned int, it will be 65536.