Yes of course just as you would say "I am one of Peter's friends"
You wouldn't say "I am a Peter's friend", that sounds ridiculous. For once I don't think they're being devious, they're just being grammatical.
Well sure, but that's not a great analogy. When JWs use that term that are not trying to say that they are a witness of Jehovah. In other words, if you are called to court to be a witness on behalf of Richard, and you arrive, you would say i'm one of Richard's witnesses.
But Jehovah's Witness is a title or name for a member of a specific religion, so the right answer should be: "I'm a Jehovah's Witness".
Likewise it's nonsense to say "I'm one of catholics", or "I'm one of accountants", or "I'm one of Yankees fans".