The page Using Security Key links to mentions that Titan supports Bluetooth & USB, with NFC support in a future update.

https://support.google.com/accounts/answer/6103523

Nothing stops promoted content being injected by the feed reader. Feedly does it on their free plans.

There was no absolute restriction on developing a competing OS (Samsung does it with Tizen for example). My understanding is that the restriction was on forking the Android codebase and shipping an OS based on that without the Play Store (you would then lose your Play Store license on all devices). Even then I'm not sure whether it's as strict as that, considering Xiaomi and others do it.