It's hard to know what, if anything, will get through to any one JW. Since you've already unsuccessfully tried to reason on JW history, doctrine, and false prophecies, those probably won't be any more successful in your next conversation.
I've never knowingly gone into a "last chance" conversation like you've described. But theoretically, the closest I've been, I would try to make the conversation as positive, true, personal as possible... and rather distanced from anything JW. If, for example, it were my cousin:
"We've certainly been through a lot together. We spent a lot of time together as kids. Remember when you asked me to explain to you what 'f**k' means? [other reminiscing goes here] Later we went in different directions. You moved away. I moved away to bethel. But whenever we got together, it was like we picked up where we left off. We're family, the ties that bind... and gag. LOL.
"One of my regrets is that I wasn't good about keeping in touch with you or most of the important people from my younger days while I was in bethel. When I finally left bethel, I tried to make the best of what my life had become. But I wasn't happy. And the harder I tried to be happy, the unhappier I became. I've told you enough of the reasons I wasn't happy and why my life had taken the direction that it has. Unfortunately, I now understand that those reasons that I mentioned to you, have made you unhappy and uncomfortable with me. I want you to be happy. So if that means that you will be happier by not talking to me and not seeing me, so be it. It doesn't make me happy to not be a part of your life, but I can't go back to living in the miserable shroud of unhappiness that I used to live in"
I'd have much more to say, of course, but I'm thinking I'd go for the non-cult persona if it's still alive in there somewhere to get them to laugh and cry. If they want to talk about beliefs, okay, I might go there, but not far. I already know what JWs believe and that they're programmed to question and attack what I would present as my beliefs. So I'd be more concerned about just trying to keep things open for future contact.
Not sure if that's any help, Anon. But good luck!