Your first question is very unclear to me, please try to explain a bit more.
For your second question, put a Conditional Branch in the common event, it's located on the first page of the event commands.
On the Conditional Branch go to the 4th page and select "Button", and set it to Z.
Your event should look like this now:
@> Conditional Branch: The Z button is being pressed
@> (Put the stuff here you want to let happen)
Else
@>
Branch End
@>
If this wasn't clear enough tell me and I'll put up some screenshots.