Hi Mike
From above as there is change in configuration values from Source into MDG Hub it is prefer to do value mapping. Key mapping is used for mapping between any objects Like material number or vendor number.But in your case as yoy want to map configuration/reference data, you need to perform value mapping means we can map ABC material type from Source into FERT material Type into MDG Hub.
Please refer Maintain value mapping via file export/import
Also you can restict users by adding Plant restrictions
