Can a Key Preserved Table have a Composite Primary Key, on Oracle db?
The problem is that we know that the join will produce only 1 row (and no more) in the view for every row in MY_CFG_TABLE but Oracle doesn't. As described in OraFAQ: Updatable Join Views (emphasis mine):


  Clearly the view cannot contain DISTINCT, GROUP BY, CONNECT BY, aggregate functions, UNION, INTERSECT, or MINUS clauses: simple joins are all that is allowed. There is one other restriction: the view must be key preserved. This means that the join must meet the following restrictions:
  
  
  The join criteria must be equals (=) joins.
  Tables other than the table that is to be updated must be joined on a primary or unique key.
  
  
  If these conditions are violated, then Oracle cannot guarantee that the view will return one row only for each row in the base table. If two or more rows in the secondary table mapped to each row in the base table, then the update would be ambiguous. An attempt to update a non-key-preserved view will result in an Oracle error.


I assume that either the joined table (which is actually a system view) does not have a UNIQUE or PRIMARY KEY constraint or one of the underlying tables (when the whole view is expanded) is not joined on its unique/primary key. This might be causing the errors when we try to update the view. 

The problem may be more complicated as we don't know the exact definition of the underlying ALL_TAB_COLUMNS view. It might have some non-equality joins or non-inner joins or unions that make the new view non-updatable.