I think I've tried that with little success. I'll test again tonight.You should have a Mercedes icon on your CarPlay app menu. If you select that you'll come out of CarPlay and will be able to use the built in navigation. To return to CarPlay you'll have to press the Home button on your iPhone. If you have fingerprint recognition enabled it should start CarPlay again.
It means having to open up the armrest and fumbling with the phone but you don't have to take your eyes of the road.