A notable mention is https://ubports.com/en/ which is different from postmarketos in a sense that ubports uses old kernels with heavy patches. That means: good support for things, but difficult future.
PostmarketOS uses the newest kernels and tries to integrate their patches into mainline kernel, so that the reliability is maintained with all kernel developers.
No. The latest changes by Google means all incremental work is now no longer visible to the public until a release is done. For most people and developers this shouldn’t make a difference.
As an example lets say I implemented features A, B and C and then did a release to v2. Before the changes you would see A get added, then B then C and then the release. With Google’s changes you will see nothing for a while and then all of a sudden see A, B, C and the v2 release all at once.