I would highly suggest you post a flow chart.
I expect you have more than one switch ?
typically in loop() you read all the inputs at the beginning of loop()
you can state that at any device pressed, you have a variable go high.
if that goes high, start a timer.
if that goes high,
turn on the display
after the last time any key is pressed, the variable goes low.
turn off the display.
no need to use interrupts.
have you been able to get anything to work yet ?
since you can buy almost every part you need, you should be able to get some parts working pretty easily.