As I suspected, you've studied all this in more depth than I have!

Can you explain how a soul can be considered 'eternal' which would mean that it has always existed, for as long as 'God' has existed? Did God not at some point have to 'create' souls, or manifest as souls? (I know that time is said not to exist at these levels.)

What you're saying about Buddhist emptiness and Brahman is very interesting and informative. I've always assumed from my reading, perhaps mistakenly, that Brahman was equivalent to the empty cosmic nothingness (the Void), and that Brahma was equivalent to the creative Oneness. So I've just done a quick search, and found this which seems to equate to my understanding: the Nirguna understanding of Brahman “is the concept that God has no shape or form”, thus emptiness? However, there were more quotes in line with what you are saying. Can you point me in the direction of any unambiguous quotes in the Upanishads which confirm your understanding?