I'm trying to work this out so sorry if I don't make any sense or there's no 'rhyme or reasoning'.
Blondie had this to say about the name a couple of years ago
Did you know that up until April 1, 1977, the WTS spelled Jehovah’s Witnesses as Jehovah’s witnesses with a lower case w? In fact, jws were counseled if they did not do that because the WTS felt it became a religious organization’s name not a description of what members did in their lives.
The second baptismal question is
Do you understand that your dedication and baptism identify you as one of Jehovah's Witnesses in association with God's spirit-directed organization?
By using a Capital W they have changed the meaning of what it is to be a witness of Jehovah, or Jehovah's witness. It now becomes a title and nothing more, I would have thought that it is up to the person/s creating that title to allocate anything meaningful to it.
I'll try playing around with the ideas as follows -
Let's say there's a bloke call Donald who has one, exclusive mistress and then accumulates a few more. They are all Donald's mistresses and surprisingly, they don't mind saying so.
Then Donald decides he likes this acknowledgement but wants to go much further and create a type of Playboy Club. There are so many that want to join up to this club and Donald wants to give them a name that conjures up a certain picture/idea in people's minds and instead of calling them Bunnies he chooses the name Donald's Mistress. They have become a Donald's Mistress, whereas his true mistresses are, individually, Donald's mistresses or one of Donald's mistresses. Now they are not all Donald's real mistresses, as much as he'd like them to be, there's just too many in this Club. However, some leave the Club. What would the correct grammar be in this scenerio?
They used to be a Donald's Mistress, they were once each a Donald's Mistress.