how custom layers can be implemented in to HERE maps in WP8? Is it possible to launch HERE Maps application from own made application and show custom POI's in HERE maps?
only way to interact with HERE apps is by using the HERE launchers: http://developer.nokia.com/resources...launchers.html
For custom overlays over Here Maps data you'd have to use the Map-Control that is part of the WP SDK to display them inside your own App. You can add custom elements to it.
Or you could implement the maps in your own application and have full control on what is shown in there.