Are the clay-like spots or freckles between the shoulder blades and chest considered an obstruction to water reaching the body during ghusl, knowing that they disappear with rubbing and were only discovered hours after ghusl, and is it necessary to repeat the prayers that were performed?

The cure for obsessive thoughts is to ignore them and pay them no mind. The congealed sweat found in the mentioned area does not constitute a barrier that would invalidate the ghusl, because it is dirt originating from the body, unlike dirt originating from outside the body, such as dust. Therefore, the ghusl is valid and does not need to be repeated.