Slowly but surely i'm getting somewhere, i hope ;-)
I think i found another solution too for recalculation the routes:
First ask the user for the destination, make it the route-end waypoint. Then get the current GPS position, make this the route-start waypoint. Calculate the route. From now on make a new waypoint that is called 'current position waypoint (cpw)'. Do a waypoint-reorder, or do a 'distance to' on the severall directions. If the cpw has a too big distance to the directions then you are getting off-track. In this case a recalculation comes in place, or so (didn't work that out). The cpw is updated on a regular basis.
At least now i don't think a 2 secs. interval for rebuilding the the whole route (start-route and route-end) and recalculation is nessecary. Also, with the cpw i can detect if a direction element is near on the track and needs to be shown. In my first prototype application the second direction element was always shown, even if it was km's ahead. I just showed the second one after each recalc, regardless of the distance and regardless of being off route.