Hey there George.
This has been an interesting thread and I have two comments.
1. I also had some issues with the RadDocumentHost. I can see what the problems is. It occurs because as PRISM moves up the visual tree to find the attached RegionManager it fails on the DocumentHost element because the .Parent parameter is returning null. I did not bother trying to go any further into the code to try to solve this... Instead (and I think that this is a good stop gap solution) all that is needed is to have a reference to the RegionManager at the appropriate place.
Personally, I happen to have a Shell structure like this.
And I have this little bit of magical code in my constructor that looks like this...
RadPaneGroupChart.SetValue( RegionManager.RegionManagerProperty, regionManager);
Which, I hope you can see, sets the RegionManager into the attached property of the RadPaneGroup. This works because as PRISM travels up the tree it finds a reference to the RegionManager, thus allowing everything to work.
NOTE: If you are doing any magic work with Scoping of regions things become a lot more messy. But for the average user, this should get them up and running and you are on your own for figuring out which RegionManager you want to reference.
2. I have been using the RadPaneGroupRegionAdapter that y'all have been nice enough to supply to the general public. It works just fine for me and I am going to continue to use it for a while longer. I have found out why the Reset is occurring (it has to do with making sure that the sort occurs when elements are added or removed), but that is not my comment at the moment.
I do not think that the methodology being used is valid. The proper way to accomplish this work should be to have a very simple Adapt method that attaches a new RegionBehaviour to the region. Then all of the work should be accomplished in that class. Much like the TabControlRegionSyncBehaviour and the SelectorItemsSourceSyncBehaviour. I plan to spend some time looking at this in the next few weeks. If I get a solution I'll pass it along, and if that is how your developers are looking at things I'd like to know what they are doing.