long story short: you can't enforce that your app opens on an incoming pns
But I think you've got the toast scenario wrong. A toast can be used to notify the user, if he taps it the app opens and you might react to that by the navigation uri (e.g.: ?fromToast=true).
If you receive a toast notification while your app is open the toast won't show. Toast Notifications only get displayed when your app isn't open. But you can react to an incoming toast notification (there's an event for that) to implement an behavior of your way (e.g.: flyout, msgbox, ...).
note: introduced with GDR3: The sound of the toast notification can now be changed (even made silent).