When is a Witness no longer "called a brother"? Logically, they would not be disfellowshipped.
If a person is df'd and makes clear that they no longer wish to be considered a Witness by anyone, how are they
"called a brother"? By who? At some point, they must return to being an ordinary worldly person.
The same would be true of anyone who da's himself. They make clear to all that they should not be called a brother. After a period of time, they cease to be called a brother by anyone.
Of course, this is all academic since the Society has no interest in the scriptures apart from the maintenance of their own power.
metatron