You mean stabilise and control the missile right? What you have written fails to make sense.
Now I am no expert at this but here is how it can be achieved. The sub goes down to 100m depth (hypothetical). The missile is pre-programmed for 100m depth. Any problems due to lack of integration can be solved through extra compensation. For example, for the test, the missile can be equipped with a depth sensor. Or it can use signals from a buoy to determine depth. For the attitude, if you read the news report, it already says the missile utilized advanced under water capabilities. Even I can think of very simple strategies for attitude correction in a test.
What people fail to remember is this is a test.
Sir, a lot can be deduced from a video. The analyst would have made assumptions about the frame rate. Then, assumptions about size of features in the background can be made and triangulation can be used to measure size. Finally, people who have in-depth knowledge of cruise missiles already have estimates of what is possible and what is not.