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.