You should just replace all the characters of 'Oblivion.esm' in all "*.py" files in Nherim\Mopy folder by 'Nehrim.esm' using an editor.
(It's five : basher.py , bish.py , bolt.py , bosh.py , bush.py)
Now, if you double-click 'Wrye Bash Launcher.pyw', Wry Bash will work on only master 'Nehrim.esm' file.