Hello jurs,
I agree with the many suggestions about having it at your home (your territory, not theirs), and have a hidden tape recorder. They'll not agree about a witness present, so why bother and raise a red flag? A tape recording of a JW Committee Meeting is always damaging because it objectively shows how weird the sect is. It won't help the outcome of the meeting, but it's great to use afterwards because it embarrasses the elders and Tower. (After all, that's what shunning tries to do to former members, so why not turn the tables on the perpetrators?)
Don't waste time trying to prove the Tower's full of it using the Bible. Anyone can use the Bible to justify crazy beliefs, so you'll be spitting in the wind with that technique. You can point out some of the Tower's many flip-flops on various issues, but the elders will turn that around with the "light getting brighter" Watchtower mind-set, and fail to see false prophesies as, well you know, false prophesies (they only see that with other religions).
Let them know you see many problems with the Watchtower, e.g., you don't wish to be part of an organization that controls members with threats of shunning, and you don't appreciate a group that shuns former members who has other thoughts about failed dates. Let them know you mean them no harm, and hope they are Christen enough to do the same, but anyone with a brain knows shunning is harmful (put the shame on them and the Tower where it belongs).
Of course none of this will change the outcome of your meeting, but a tape recording of their bazaar Tower-thinking can be useful. Lots of luck furs; let us know how it turns out--and we would love seeing a transcript of the tape, that really ticks the Tower off!