Google Hangouts on my Android phone keeps bugging me to intergrate SMS into Hangouts, as well as to "confirm" my mobile number. My fear is that something analogous to the iMessage "purgatory" might happen, though I haven't heard of anyone experiencing it.
Google isn't doing #2. The SMS integration with Hangouts is limited to just showing your SMS and IM messages in one view. When you send a message, you send it through one channel or the other explicitly. Google doesn't ever guess how you wanted to send it, so they can't guess wrong.