Dear Lisa,
I completely agree with Monica on all the points she made. Some modifications that I would suggest for this is to play around with having the toes curled or uncurled. I understand that the author of this book is making certain points, however I don't agree with the idea that a posture has to be done the same way in every body, simply because no two bodies are the same.
Another modification that I would almost insist on is keeping the gaze forward and not tilting the head back. By keeping the chin down that will help to get the shoulders to move down the back AND it will take the unnecessary pressure off of the cervical spine. I'm of the very strong opinion that there is absolutely no need to drop the head back like that in any posture. The neck muscles need to be strong in order to reduce the strain there on the spine, and sadly most don't have very much strength in the neck. Doing this to the neck can cause subtle damage that can cause pain or aggravate any existing conditions of the neck.
A way to strengthen the neck is to practice camel without dropping the head back. Instead keep the neck long with the gaze on the ceiling. Being in a back bend with the neck actually working to keep the head held up with strengthen those neck muscles.
I like modifications and I teach as many of them as I can get into a class because a little modification can go a long way for the student that needs it. I have a hard time with any instructor that insists on a posture being done in only one way.
These are just some thoughts to add to the conversation and to your considerations
Deborah