I'm a lawyer and were I counsel for the Witnesses I would not be afraid at all. Having not read the legal strategy, I can't comment. With what I know, it is inconceivable to me that a court would police relationships within a religion. It is exactly what courts are forbidden to do. All I can think of maybe having a chance is a libel, defamation action. Maybe if they were so intrusive making allegations that you lost a job. Henry VIII launched his infamous law suits against his lawful wife. It was the very thing that Founders were insistent be stopped in this country.
I would be interested in hearing the theory. There is always the theory that any legal letter might have a chilling effect. You don't stop them from practicing their religion. Heck, the Supreme Court ruled recently that the Salvation Army could practice hiring discrimination as part of their religion. They had new light that all workers should be members of their religion. They fired people who refused to join the religion. Antidiscrimination statutes exist. The First Amendment trumps even discrimination statutes.
Besides, aside from business loss, I don't see what damages or remedy any court would order. If they aren't sophisticated, they might be afraid. On the other hand, threat of legal action inflames a situation. You would likely end up with a far worst situation. Broken hearts and souls don't count in law. Law does not guarantee justice. I see little justice in law. I would think not being afraid of them, not recognizing their legitimacy and talking to them as someone entitled to certain human rights might accomplish more than legal letter.
Perhaps I am not aware of something. This is my knee jerk reaction from all I've ever been exposed to in First Amendment law. I'm not as strong in torts such as libel and defamation.