When in conversation with JW's those questions, or varients of them, often come up.
I have noticed though, that because of their fear of any challenge to their faith, they do not wish to hear the answers, so I have developed an answer which at least I hope accomplishes a couple of things.
What I say to the first question (why did you leave ?) is "Because I am a lover of Truth", to the second,( will I come back ?), somewhat similarly, I say "As a lover of truth I would need it proved to me that you have the truth".
Both responses show that I do in fact challenge what they believe, and that I did not leave out of "weakness", nor simply to enjoy a life of sin, Mrs Phizzy would never let me anyway.
The last JW's I said this to, I actually ended with "there's a challenge for you !".
Of course, I don't expect to find myself a "Return Visit" any time soon.