other element above mapview

Isn’t there any chance that an element with its children to be displayed above a mapView without being the mapView’s child?

You can have that. The only requirement is for that element to be inside of the same NativeViewHost as the MapView:

    <Panel />
    <MapView />

My issue is very similiar to this one, so I don’t want to open a new thread for this.

I have a Page within my Navigator and below a Grid for my Navigaton:

        <!-- Some Content -->
         <MapView />

<Grid Alignment="Bottom" ColumnCount="2">
  <Panel Column="0" />
  <Panel Column="1" />

When scrolling through the content of my page, the MapView is being placed above my Navigation-Grid.
Is there any solution in solving this without wrapping the NativeViewHost around my complete structure?

No, a NativeViewHost will always be rendered on top of everything else.