An interesting question.... I think if truly entrenched witnesses were able to listen, to actually use reason like the rest of us, they probably wouldn't be witnesses, lol.
I personally would have a hard time even engaging in such a conversation, as in my opinion, their bible basis for everything is moot because of the realities of the bible itself. And no witness will EVER admit the NWT is bunk, let alone the bible itself being less than the lifelong guide book they all live and die by.
On the reasoning side, it seems pretty simple to me. If jehovah wanted us all to be robots essentially, he would have created us that way. (I think I just threw up a little in my mouth....) He certainly wouldn't have given us all the range in personalities and emotions that he has. Literally, in our DNA, we're not all the same. Why would he purposefully make something more difficult for us if that's not what he really wanted? After all, it was Satan and Eve that have gifted us with our difficult situation in these last days, right? That certainly wasn't Jehovah. He's simply letting their bad decisions play out.
So why would be purposefully create such an obstacle, that makes service to him that much more difficult?
(oh man.... I feel nauseous now, gross)
And THAT'S why you'll never have a true debate about such things with a witness. Because it opens up other doors that they don't want to look in. Once you start looking at things from a rational perspective of what a god with such power would actually do, you start to see all the holes in that kind of perspective of "god" period.
Its a slippery slope for a witness.