normal raid 5 implementations do not require that you read all the blocks to update one, they only require thatyou know the old and new contents of the block that you are modifying and the parity block. so that isn't a ZFS advantage
I wasn't aware that ZFS could mix raid types/configurations within a single filesystem. that seems very strange to me, are you sure that it can do this? I thought that when you created the ZFS filesystem you told it the raid type and configuration to use.
the basic problem is that wuthout doing a sync, the OS has no way of knowing when the data written to different drives actually hits the media and is safe (and this can happen in different orders for different writes, even to the same spot), thus the need to do 'raid journaling' or similar.
I don't believe that ZFS does COW, doing that would fragment the data very rapidly, and that is death to performance for drives that need to seek.
I don't believe that your two points are valid in themselves, and even if they were I don't see how they combine to solve this problem.