Upvoting since you put time into this!
I think you overestimate how much control steemit.com has over the content that shows up here. If steem continues to be successful, advertisers are simply going to buy steem power and use that to push their ads to the front page.
correct, it is decentralized only to an extent. Anyone with enough $$$ or enough hired big boob models can easily gain enough steem to monopolize the trending feed.
an "official" advertising square might not even be needed at all!