Hmmm, I just tried to connect using both the iphone and ipad apps on my devices and the chromecast icon doesn't appear to be showing up anymore, regardless of the game settings. I haven't tried to use it for a few weeks. Maybe they removed the functionality temporarily? I apologize for leading you to spend so much time looking for it. I promise it was an option a few weeks ago. When it was working it appeared next to the (?) in the top right of the screen.
It also looks like the auto-detect wifi game feature has been removed as well. That's a bummer. This could be a indication that these features are being worked on. Can someone from SMG weigh in on this?