Alright, so I managed to dig up some of my sources for Zunist/Kalash related stuff:
I had some sources concerning the practices of the Nuristani in Afghanistan but couldn't find any. I would suggest googling
The Kafirs of the Hindu Kush by George Scott Robertson, which is the primary Western academic source on the pre-Islamic Nuristani as it was written right on the eve of their conversion in the late 1800s.
This also reminds me I really need to post those suggestions for Zunists soon, which you might find helpful as it's also basically just Kalash/Nuristani practices put in under the Zunist umbrella (given I interpret Zunism as a catch all for Indo-Aryan "paganism" in Afghanistan and northern/western Pakistan).
Concerning the layout, I'm also a bit iffy on calling them Nuristani in-game, as the term was given to them after their conversion to Islam in 1896, per the Encyclopedia Iranica article above. However, if no other good term can be found, I can understand its usage as one of convenience. I also recommend just using Encyclopedia Iranica in general for a lot of stuff - even though it chiefly concerns the Greater Iranian world, it has quite a number of articles related to the Indian subcontinent due to the long history of contact and influence between Iran and India. It is written by academics but fairly accessible, especially if you already have some basic background (only problem is its search function is absolute crap last I checked, so I suggest using google to find articles from it).
Otherwise, I have nothing much to say at the moment, but out of curiosity, do you know if we are able to do it like in CK2 where you can put new cultures on a separate file but still have them added to existing culture groups? Or do we have to directly modify the existing culture groups? Reason I'm asking is because if I have to do this then it could be a compatibility headache for my mod (not with your mod per se, I have no plans for anything in India, but in terms of modifying vanilla code).