it is ok that the signal dies when you arent standing on the island anymore,
we want to measure the speed/force used to press these buttons
Yes but I also said that the signal dies when you continue to stand on it, it is only at the moment of impact you get a signal.That means you need an analogue measure. That will be hard even with the reduced number of sensors because you won't have time to scan them all in the time the signal lasts, so you might miss some.Using Piezo sensors is not the way to do this project if you want it to work. I fear you have underestimated the difficulty of your task.
The address lines are created by the arduino to select which signal is going thru the multiplexer.How quickly do you expect these sensors to be pressed? 10,15 at a time? Or just 1 person walking around?