David: You raise some very interesting questions on a topic I haven’t thought about for quite some time. If you don’t mind, I’ll take a little time to think about them and then get back to you.
Before I do, remember there are two ways to criticise an argument: 1. dispute its validity (claim that even if its premises were true, the conclusion wouldn’t follow); and 2. dispute its soundness (grant its validity – at least for the sake of the argument – but dispute the truth of one or more of its premises). You’ve asked about the shape of the argument (validity) and disputed its soundness by questioning premise 3.
Thanks for letting me know that you opened up a new thread.