Please don't miss the fact that the main reason for mirroring the dict table is to get O(1) node lookup (in the linked list).  Otherwise most lookup-dependent operations, like __delitem__(), would become O(n); whereas in the pure-Python implementation they are O(1).  This is all explained in the notes at the top of Objects/odictobject.c.

Also, I didn't change anything in the dict implementation to rely on the OrderedDict implementation.  So while I would say OrderedDict is coupled to dict, I wouldn't say the reverse, that dict is coupled to OrderedDict.  If dict changes then OrderedDict must be updated apporpropriately, but not vice-versa.  That should still hold.