I may not be following you, or fully understanding your problem, but I think I have the solution.
I had not seen a problem myself, so I think my intuition and yours may be different, but here is what I do.
Having selected the destination and ok'd it, the map appears.
I then go to radio or media player.
When I want to return to the sat nav system, I DO NOT touch navi, but touch MAP.
My current location and the route is displayed, and I can see and hear the directions.
I can switch between map, mediaplayer and radio, without loosing the mapping and route details.
There is no need to use NAVI again, once you have set a route, unless you wish to change the route or destination.
Hope this helps.
Edited by: crossbow