If I recall, the letter was not spectacular. It would not impress another lawyer. I can see where it would impress someone not in law. My thought is that lawyers get paid for writing letters. Elders may shrink at such a letter. There is no guarantee, though. If they forward it to the legal department, they will be vindicated. The factual details are very important. I often find that clients frequently omit the key facts b/c they don't know what the elements of a tort are. On the other hand, you have to consider the inflammatory qualities of such a letter. If only you are involved and very certain of your beliefs, it may be worth a try, IMO. If you are talking about family or friends, though, it may backfire.
The solution that I would endorse is not legal but very potent, IMO. Living well is the best revenge. As Jesus instructed his discipiles, wipe the dust off your feet and move on to a better life. They do have a legal right to regulate their religion and not associate with someone they don't want in the religion. NO court is going to rule on the merits of a religious decision.
A secondary solution would be to be very vocal about the shunning in all its sick details. Last night I watched a couple of shows about Amish shunning and Amish leaving the order and facing a new world. Next, a similar program aired about the Haisidim. If you think former Witnesses have problems.....
Unless there extra facts, a letter concerning a routine disfellowshipping would make me laugh. I don't know how the elder/Bethel connection runs. It seems to me that we should be allowed to shun the Witnesses in return. I would not bet on settlement details unless I read the settlement myself. This is why seeing a lawyer for a consult may be very important. Your local elders may very well have overreached and crossed the line. Internet advice is bad advice.
Most consults are free. These are hard times for lawyers so they will be generous with their time to land a client. You may get ideas or encouragement here but nothing has the comfort so much as being able to attach a legal malpractice policy if the lawyer screws up. There is also recourse to the state bar administration. Something you believe is insignificant might land you a large award.
I recall the letter so it is archived someplace. My sense of time is not good. If I recall, it may have been posted a while ago and then bumped up. Disfellowship/legal would bring many hits.