I think that, for the most part, khukuri manufacturing is the same basic skill throughout Nepal and other parts of that world. Are there differences?........ sure, but the basic principle is the same, and that can be caught on film without exposing certain details one would rather not share.
If there's a deviation that one would rather not share, one would just leave that aspect out of the film. Just mention during the film when a deviation was left out, and why (for the sake of not sharing that certain info with competitors). As for the types of tools being used by HI, that part can be added in, since it's no secret that they have adopted the use of some modern electric tools and such (no letting the cat out of the bag there).
The basic hand forging of steel, hand forging of brass, handle shaping, blade shaping, sharpening, polishing, leather covering, component installation, and other common routines, should be easy enough to film without causing any problems whatsoever, IMO
Yangdu and her father could easily leave out whatever they felt was too sensitive to expose, though I doubt that at this point in the khukuri timeline there is much, if anything, that nobody else knows about or has not tried to do somewhere else in Nepal. It's more about what "they" (HI) chooses to do and use, rather than what the "others" (their competition) are truly in the dark about it........... IMO.