Those scriptures you refer to about God punishing children for the sins of the parents are being misinterpreted by you, I think. I think the verses say that the sins of the fathers will be visited on the sons even to the third and fourth generation. I think that means that when parents don't do the right thing, it has an effect on the children and grandchildren.
For example, if parents don't work and provide for their families, then they go on welfare. They don't get a good education, and it's almost impossible for them to do any better, and on down the generational line this holds true.
I'm not disagreeing with you about how you raised these questions for the JW, but I just wanted to point out that some of your arguments may not be as cogent as they could be. I'm surprised the JW didn't explain this problem the way I did. I don't think they know much about the Bible anymore.