"Can it be forked, cleanly?"
Sure, just like Android. How far any fork has gone?
Moreover once they are no longer bound by GPL and someone else's copyright on code, they will have much more freedom on what they open source and what they don't.
It's the Apple model - only parts of macOS are open source, those parts they think its advantageous for them to open source, while keeping closed all those parts which they deem dangerous to their business to open.
Make those parts essential for any usable system to work, and remunerate developers who write apps for your system.