‘Loading Accessories and Scenes’ ⇥ support.apple.com
From an Apple support document about troubleshooting HomePod setup problems:
Open the Home app on your iOS device and check that you see your accessories and scenes. If you see a message that says loading accessories and scenes, wait for the Home app to finish loading. If the Home app stays in a loading state for 30 minutes or longer, you should see an option to erase and reset the Home app.
First of all, I think the timeout for triggering this debugging mode for HomeKit accessories should be much shorter than thirty minutes.
But, as someone impacted by this problem since the developer betas of iOS 10, I kept my iPhone awake and running the Home app for half an hour. Twice. Both times, I did not see any option appear that would allow me to reset the Home app, its settings, or anything in iCloud. I also cannot find any additional options in Settings to reset any data.