Currently, every change to a Path object using pen and direct selection tool is just an UpdatePropertyCommand with new StreamGeometry passed for Path.Data property. This makes it hard to track changes done to the path view model.
Wrap these operations in a separate commands (e.g. AddPathLineCommand, CurvePathVertexCommand, CurvePathSegmentCommand, etc...). These commands can just call on UpdatePropertyCommand functionality, but should modify Path view models directly. Consider moving PathEngine from being local to a tool state to being global and defined in XamlDocument.