Great points. I think that helps me fill in a few gaps in what Samsung might be thinking here.
Google has threatened that Samsung lose support if they fork Android. They might or might not actually do it - Samsung is a major Android player. The threat is still enough that Samsung have chosen not to try forking Android.
However, Samsung still wants more control. The solution is to use Tizen with Android compatibility to try and wrest that control from Google, but while still acting within the boundaries Google have given them. If Tizen doesn't take off then Samsung can just fall back on Google Android. If Tizen does take off, then they now have enough weight to push Google around as they wish.
Google isn't worried about Tizen as they believe that Samsung will not be able to out code them, and that Android apps running on Tizen will not be as good as Android apps running on Android. While I'm sure that Google would prefer if Samsung wasn't trying out Tizen, they believe they have a strong enough bet that Tizen will fail and Samsung will come crawling back anyway.
So the whole situation now makes a lot of sense from both Google and Samsung's side. From this we can probably take away that Tizen will get the best hardware first to give it more chance of taking off. Also, Google is likely to try and add new services in future Android versions that will make it more difficult to run Android apps on other platforms. Interesting and falsifiable, in a year's time we can see if these predictions are true!
Anybody think I've got way on/off track here?