So let me see if I understand
A person can only be shunned if they are fake Christians. For example ..Bro. Smith is trying to remain in the Congregation but won't give up immorality. He should be shunned according to this scripture.
But once he leaves ....you can resume association with him.
That does not seem right to me.
The point is that if the person is professing to be a christian ("called a brother") then his conduct reflects badly on the congregation and members stopping normal association with him signals to him and others that he is not living up to what it means to be christian and his actions are all on him and should not be attributed to what the congregation stands for. They're basically showing him up as an imposter, disavowing his claim of membership with them.
But once the person is no longer professing to be a christian then there's no longer any danger of his actions being attributed to the congregation. They no longer have to take extraordinary steps to show him up. He's now just another non-christian and not an imposter tarnishing the name of the congregation and its standing before god.
I Corinthians 5 is dealing with persons who want to continue to be a member of the congregation and have all the privileges of the brotherhood while living in gross sin. it does not apply to someone who is no longer a member and no longer wishes to be considered as a member enjoying the privileges of membership.
So scripturally, a person who disassociates from the JW organization and is living in sin should not be treated any differently from a never-JW living in sin.