Didn't read all the way through this thread, but here are a couple of facts about trademark law that may be relevant:
1. Under the first sale doctrine, once an authorized product (i.e., a product made by the TM owner or made under a license from the TM owner) is sold, the TM owner cannot prevent that product from being resold. So if Nash is buying officially-licensed necks, he can assemble them into basses and resell them. For the same reason, the OP could buy a Fender-licensed neck from Warmoth or Allparts, build a bass with it and then sell that bass, and Fender could not do anything about it. The first sale doctrine applies without the need for any separate agreement with the TM owner. There are some nuances to the first sale doctrine that could complicate things a bit, but that is the basic rule. The attorney who wrote the original letter to the OP may not be aware of where the necks came from -- i.e., whether they were acquired from an authorized Fender licensee -- so that could be part of the issue.
2. For all of those telling the OP not to worry about Nash, the fact that there are third parties such as Nash using the TM at issue (or, in this case, the trade dress of the headstock) could in fact be highly relevant. If a TM owner allows third parties to use its TM without authorization, that TM owner can in some cases lose the ability to enforce its TM. In other words, if Fender ignores Nash (and perhaps others), that could make it harder for Fender to stop the OP from using the headstock shape. Again, there are nuances, but the OP is right to raise the issue of Nash's third party use.