It does not surprise me that you are better versed in Watchtower theology than I am, TD, but as I do not have access to the private letters you mention I cannot really comment on them. But it does seem clear to me that Jesus was specifically saying that their having swords was a fulfillment of prophecy. After all, you wouldn't identify someone having a money bag as lawless, or someone having a food pouch, but you may well identify someone having a sword as being lawless. Especially if he was resisting arrest or cutting off ears.
Your reasons that it is implausible that Jesus was talking about the immediate situation rather than a more general preparedness are all good (if I have understood you correctly), but his disciples thought he was talking about that moment because they said "Lord, look, here are two swords". And his reply "It is enough" also refers to their immediate situation. I dunno. What Jesus says doesn't always make sense to me. But in this case it does show how his disciples understood him regardless of the assumptions it leaves in its wake.