Your spine is flexing a little. More than I would like to see. You are also correct that your elbows are too high. Your hip mobility is likely fine, however. Here are my recommendations. Widen your stance an inch or two. Drive your knees out harder. Drop your elbows. All of this should make some room for you at the bottom and encourage you to stay in normal extension. As an aside, are you able to voluntarily overextend your lumbar spine while it is unloaded? If you do not have control of your lumbar spine, there is a little section in the book that goes over how to do the superman exercise to show you what you need to be able to do.