Hey there, I know I’m coming in way late on this but I may have found an easy solution. I had 38 products not approved because of the missing country sub attribute. Implementing the following fix immediately made them approved. Just go to the YouTube video link below and this guys walks you through the simple fix. He shows you how to easily add a rule that sets the country as US if it is not submitted by your Shopify content API.
https://www.youtube.com/watch?v=2uNrJIrOQFA
He offers two methods. Skip to 2:33 for the method that worked for me.