Finally fixed the navigation! Yippieh!

Ok, so what is the magic?
I've added another route for the masterPage as subpage of the splitApp. This is called on navigation from tile to splitApp.
On non-phone devices, the masterPage triggers the load of the detailPage as well (via MyNavWithoutHash).
On phone devices, this is not done and the display remains on the masterPage.
Additionally, back navigation on detailPage is adjusted to masterPage and not to splitApp anymore.