As you can read at http://www.goxam.com/2.2/helpWPF/webframe.html#Northwoods.GoWPF~Northwoods.GoXam.DiagramPanel~InitializeLayers.html, the “Tool” and “Adornment” Layers are in front of the “Foreground” Layer. This is what most applications would want, since normally one would not want Adornments to be obscured by any Nodes.
But if you do want that, are you sure that node 2 is a regular node? Is it representing model data and is it interactive in the usual way that nodes are in your app?
If it is, perhaps you want to add a special Layer that is in last in the list of the DiagramPanel.Children. Or if you are not using the “Foreground” Layer for anything else, just re-order the standard Layers there, removing the “Foreground” Layer and then immediately adding it back to the Children.
If it is not really a normal Node, perhaps you want to make it an Adornment.