A notarized letter is a great idea but as phizzy said "if you threatened legal action, you must intend to go through with it"
A average JW would probably not have the funds to follow through with legal action and the elders will also know if the person had the means to see it through, if they do have the money they would possibly accept the letter and do nothing.
And the way JW lawyers have treated others that have taken the org to court we can be sure that they will be as difficult as possible to increase the costs and drag it out as long as they can, and if they lose they will appeal to drag it even more.