I don't quite buy your argument. Sure, nobody is forced to use Android, in the same way PC manufacturers aren't forced to ship PCs with Windows installed.
You're making it sound like there are compelling alternatives to Android for a handset maker, but really there's not.
Sure, Amazon can fork Android, but can they contribute to Google's version? Can they collaborate and make decisions together? Not under this governance model...
Can just add some software to Linux? I don't think so - Linus calls the shots... I don't think an OSS project where everybody could contribute would work.
Yes. Pretty much anyone can contribute. Your patches just have to go trough an approval process. It is complex and takes time, but of course it is possible.
If the patch contains good code/documentation, doesn't break anything, works as supposed to do and is relevant to the direction of the project, I can't see why it wouldn't be approved.
If my patch conflicts with the direction of the project, I can just fork.
And no, I never tried because don't have a device running 2.x. I could try contributing to Honeycomb (since I happen to own a Xoom). Is there any repo I could clone containing Honeycomb? (honest question)
Nobody is forced to use Android. You could always license Symbian or something...