I try to let NI work with RTW 1.5 (no BI 1.6, no RTW Gold).
You need to create an empty BI file within your RTW file, to put the NI file within the BI file and to change the three data files below.
I should be able to play the campaign through the provincial campaign, but when I try, I am told bi/data/models_strat/BI_strat_diplomat.cas doesn’t exist. Help is welcome.
I haven’t tried to play a custom battle, so I cannot tell if this is possible.
EDIT: The trick that worked for RNJ doesn't seem to work for NI.