You do forget one 'small' thing. If google decides to make that app and include it into EVERY android, OEM's will have to play ball. Why do you think that every android comes with play store, chrome, photos and some other google apps?
Google demands it. Otherwise, there will be no google services on that phone, and that means no play store. Even Samsung without gmail and play store would fail, not to mention others like HTC, LG and Sony.
You mean like Amazon, right?
You're incorrect. Google makes no such demands. They offer guidelines, OEMs accept or reject based on whatever they're trying to accomplish with a given handset. This is why Samsung has created its own duplicate ecosystem on all of its handsets, it's why OEM's have gotten rid of the app drawer if they so choose, it's why updates are controlled by OEM's and carriers, and as I first suggested even the Play Store isn't a demand.
Google isn't going to be insisting on one standard any time soon. Their interest is in having their software on as many devices as possible in whatever form they can get it on there. Everything else is a distant second.