Using several Arduinos simultaneously
Among all the options mentioned, using a master/slave should be the last resort, especially when you are new to embedded programming or Arduino, it is not only more complicate but also more expensive to implement.
One potential solution that could free up the precious digital i/o pins that were used by the 5 buttons is using analog input to detect the status of the buttons.



simulate this circuit &ndash; Schematic created using CircuitLab
When sw1 is pressed, the voltage at the A0 will be around 0.83v, and when sw5 is pressed, the voltage at the A0 will be 2.5v. You can write a simple switch...case to determine which switch has been pressed.
The only problem of this circuit is that it can't handle more than one button been pressed simultaneously, but that could be solved by double the value of each resistor like 10k, 20k, 30k...
This will free-up 5 digital pins that can be used for control (such as servo, relay, etc.).