Instance Properties prevent me from editing Instance-local Properties
        
          Summary: Create a Model with some Properties.  Set its Instance Properties appropriately.  Create an Instance, and set one of those Properties.  Delete the Property from the Model.  It is now orphaned -- since the Model has Instance Properties set, editing the Instance doesn't let you edit the Property, but it's still there.
The implication is that, if an Instance has locally-defined Properties that are not defined on the parent, they must be included in the Editor, regardless of the value of Instance Properties.