If I am understanding correctly, I believe you should be able to set a data limitation on your Regions dimension. To do this,
expand the drop-down menu on your Regions dimension > under "Limitations" select Fixed Number > choose "Top" > and in the formula bar put "21" (as this will display the top 20 plus a section for "others").
A simple option to display values for only sub group "NM" would be to use the measure:
Please let me know if this helps!