I decided to try and improve the look of my Sandbox3D by opening and styling it in Expression Interactive Designer (EID).  I'm still in the process, but three discoveries I made which others may find useful.

1. EID has a runtime XAML parser that displays very useful debugging information that you don't get w/ the compiler.    Because EID has a WYSIWYG interface it does it's own parsing and displaying of XAML and, if you end up with some XAML it can't deal with, it will provide its own errors for you that you may not get from a compiler.  For example, when it tried to parse my XAML, it found mistakes in my databinding code, displaying them as follows:

Takeaway: EID is a useful tool for taking existing XAML and helping you debug problems

2. EID has two methods for displaying the tree: mark-up order or z-order.  At first, when I pulled my XAML in and corrected my errors from above, my tree showed up with the last element in the XAML appearing first.  This, to EID, is z-ordering.  You can toggle this view by clicking on that text to get mark-up order instead.

3. EID can show you the default style in XAML.  Way back when, I posted a way to serialize the XAML for the default style WPF provides.  Those APIs are deprecated, but you can do the equivalent in EID but selecting "edit a copy of the style" from any control.  This is a great way to (1) understand how styles works in WPF and (2) create a foundation for your own custom styles.