this is the first I've heard anyone suggest that Satoshi was unaware of Szabo's Bit Gold. What makes you say that?

I think if he had been aware, he would have cited Szabo in his paper, like he did with b-money. And yes, Satoshi learned about b-money from Adam Back. Here's part of Satoshi's email to me:

I was very interested to read your b-money page. I'm getting ready to release a paper that expands on your ideas into a complete working system. Adam Back (hashcash.org) noticed the similarities and pointed me to your site.