Sapphy is dead on.
Can I ask exactly what this letter you signed said? If it just said that you recognize that he is "scriputally free" (code language for admitting that you've had a sexual relationship with someone), without going into any details, you may be able to argue that 1) you didn't understand what you were signing, or 2) that because of his previous abuse you were coerced into signing it.
Your ex obviously wants to date (or is dating) another JW and that's why he's doing this. But, if you don't want to get df'ed then you would pretty much have to play by their rules and tell them how terrible he was because they will take that letter as a confession and if you're not there to defend yourself, it will be a 2 minute meeting.
I like Scully's idea of going after the individual elders for slander but that's going to cost money. Putting it out there may make them back off. I personally would love to see someone do that because I know the WTS would not defend their individual elders.
Of course, the whole thing is clearly unscriptural but then again, that's the WTS for you.