Some great questions here in your post - thank you
I also posted a range of questions having researched into the fundamentals of Steem, Steem Dollars and Steem Power a little while ago. It would be great to get more insight into the mechanics of the machine https://steemit.com/steemit/@knowtheblock/steemit-questions-somebody-help-me
RE: What is the size of Steemit.inc's Steem stake? Does it present a risk to the Steem economy? What is being done to mitigate the risk?