Thanks for writing.
Yes, the backbend over a large ball can be a great yin posture...for some people! Notice how Donna's body is in full contact all along the ball. Other people with more limited ranges of motion for spinal extension would not be able to do this. I would call this “track 2 or 3,, which would be great for more flexible students. For students with less range of motion (track 1), who are stopped due to compression of the spinous processes, and who are not in contact with the ball, they would be continuously relying on their compression points to support them rather than the ball. This may not be bad, but they would have to check to make sure it is okay for them. For less mobile students, I would suggest starting with the yin version of the Bridge pose (see here
.) However, theory only goes so far...in your case, your reality is more important. If you find that this feels good and helpful for you, with no pain while in the pose, coming out of the pose or in the next day or two, then - great! Keep doing the pose. Enjoy it. For the days you don't have a ball handy, and you don't want to do Cobra, try Bridge.
Regarding counterposes: I have written about them here
. The short answer to your question of “are they always necessary?, is “No!, Nothing is always necessary. Again, it comes down to experience and intention. If you are planing to engage in some sports or yang activity afterwards, however, I would strongly suggest counterposes to reduce the creep created during your practice. If, however, you sense no creep has crept in, then you are probably fine to skip the counterposes, as long as you still have a proper shavasana at the end. Again, rely on your own judgement and experience to guide you more than anyone's theory. What is your reality?