I just checked, and although I can't find the capital of Cherokee, I do own the potential capitals of Creek and Shawnee. None of them still exist.
Is it possible I just have too many potential releasable nations? I have over 20 from my conquest of India and Indonesia. However I don't think that's it, as European Portugal itself is releasable, and it was gained at the same time as North America.
Also, after looking through my releasable vassals, it appears that I can't release the Inca or any of formerly Portuguese Africa.
No, this is working correctly. The changed culture in their former provinces is the reason you can't release them.
They will change religion and culture, as changing from an pagan religion changes the province's culture. I -think- the tech group will change to your tech group, although their tech level will be the same as when they were annexed.
No pagans in North Africa.
Also I don't think the tech group will change.