Apologies Walter, having looked into it further, I removed these invisible panels just to test before trying to make them into an adornment, but the link are still taking weird routes with the same setup.
The real reason here seems to be the AvoidsNodes router which is unfortunate because it is otherwise useful.
Another thing I tried was to increase the columnSpacing and layerSpacing on the LayeredDigraphLayout - that does seem to have an effect but not quite what we want.
Does AvoidsNodes use the whole node or some object inside it like the selectionObjectName (does not seem to be that one)? The avoidable property is not available on the TextBlock it seems.
If not we might end up using Orthogonal anyway.
I did also find this post that states that AvoidsNodes does not include the two nodes that it is connecting, but that does not seem to be the case here - perhaps it has changed since then?
Any advice?