I honestly don't think Amazon will take that risk - their changes are going to be cleanly designed such that they can pull Google's AOSP code, merge their changes and crank out their own updates. I can't see Amazon developing 2.1 code base without merging Google's bits from newer releases - there is just no way they are going to be able to catch up with all the stuff Google and co. are putting in Android - they will never have that kind of expertise unless they hire people from Google/Android team which I find somewhat far fetched.
Keeping up with the releases from 2.1 and going to 4.0 is going to be a huge pain. And that will be required for compatibility sooner or later.
Nothing will happen. Kindle Fire users will continue to read their books, watch their movies / tv shows and browse the web as before. This is being marketed as a Kindle for a reason.
Amazon's a pretty big SW company. I wouldn't be surprised if they started their fork with 2.1, but will bare no resemblance to it. Amazon may have their own internal Android fork built up now and may not merge back with the Google version of Android.
I don't think Amazon wants Google to control their destiny. And considering what Google has done in their 3.x branch (none of which is very impressive), Amazon may well believe they can out execute Google. And considering how few Android tablets have shipped, Amazon could really quickly dominate the Android tablet market -- and then be the one calling the shots.