They're now threatening to withhold early access to new versions unless the manufacturer adheres to an upgrade pledge. I doubt it will work.

Google absolutely needs to do some curation, but that doesn't address the fragmentation much if at all. There's no walled garden, so there will always be competing Android stores. What we'll probably see is Amazon and Play duking it out for the top 1 and 2 spots, and multiple much smaller stores from the manufacturers, carriers, and so on.

There's probably room for a 3rd party in-app purchase provider, but the apps will need to include the libraries themselves or users won't bother. Theoretically you can use Amazon's in-app purchasing for apps sold in the Google store, but only if the user downloads and installs the Amazon store app. Google Play requires Google Mobile to be installed, which is a bit deeper than just an app. If you have a Kindle Fire you have to root the device to get it on there. The vast majority of users, again, aren't going to bother.

The Nook doesn't even have an in-app purchase API for its store, and in fact B&N will reject your app if it implements in-app purchasing or advertising.