Agree 100% with Steel in getting an equine vet (pref specialising in lameness rehab) involved. 'Nebulous' is an interesting description and one I also tend to agree with, because until recently there was very little understanding of the processes that led to 'navicular disease'. Now there is a lot more understanding of what goes wrong/what is needed, although more research is definitely needed, and being recent 'discoveries', much of it as yet anecdotal, many 'experts' have apparently not yet considered a lot of it. It also appears to be a whole foot problem, so only considering the immediate vacinity of the navicular region tends to lead to very 'nebulous' views about it.
Considering the above, it is vital IMO to do your own research into current research & alternative treatments, because it does seem to depend on different 'expert' experiences/attitudes as to what they will even consider, so you may decide that despite what a particular vet says, you feel a different approach(& different vet) would be best for your horse. Eg. Traditionally, until relatively recently, 'navicular disease' has been considered as incureable & so conventional treatments tend to be solely palliative and due to not treating the underlying issues, conditions tend to worsen & palliative treatments also tend to be temporary.
I do not discount the potential value of conventional treatments, as depending on the severity, time frame, age of the horse, etc, providing a little more pain free life may well be the best that can be hoped for in some cases. However, as there seems nothing to lose & everything to gain in attempting actual rehabilitation, I think the palliative approach should be saved as more 'last resort' type practice.
'Nerving' a horse involves severing the digital nerves in order to make the damaged foot insensitive. As it does nothing to treat the problem, removing the feeling without treating or protecting can lead to further injury and the cut nerves can also develop painful tumours & do tend to grow back, requiring further operations, I personally consider nerving to be a very last resort palliative treatment of old & truly incureable cases, to give them a little more painfree time. You can check out this article, for one renowned vet's opinion on the procedure; petArtistWithPeaches horse blog » Dr. Teskey on nerving the navicular horse