Very good question. And no worries about bringing me down haha, I'm always up for a convo about psychology even if its about something like this.
For the first part of your question I have this to say. Psychology is a new science. When medicine was 100 years, old doctors claimed that diseases were due to evil spirits. Obviously the lack of technology and such played a large role in that but what I'm getting at is that psychology will evolve and become more scientific. Right now it is pretty theoretical. There are some flaws about psychology though, as with anything. Its just much easier to exploit them since psychologists can have a negative connotation. Psychology is getting better though. Much better. I have read a lot of about early psychology and I compare it to today's research and it is simply awesome to admire such progress. Early on psychology was pretty much all theories. A person would get a PhD or MD and more or less create notions of what they think psychology is without testing it. Throughout the 1900s psychology did start to become more objective and scientific. It is still advancing and still has flaws but I believe that psychology will continue to progress and become more scientific.
This leads to the second part of your question. Naturally, if a science is too theoretical different scientists will disagree because there is no firm standard. So, in diagnosing a client or patient, psychologists can formulate very different answers. Though doctors sometimes do this as well but it is much less common. I was reading a book by the great psychologist Albert Ellis. He was saying that when he first started doing psychotherapy, he used the psychoanalytic approach. Freud made this very popular and most psychologists used this in the first half of the 1900s. However, Dr. Ellis wrote how he realized that psychoanalysis was not very successful at all. Sometimes he would do everything right in therapy and the client would still be not cured and sometimes he would do many things wrong according the proper psychoanalysis and the client would be helped. Thus, eventually Dr. Ellis created a new type of therapy known as cognitive behavioral therapy which is today a proven therapy for treating phobias and eating disorders. This example shows that psychology is evolving and better more consistent therapies will arise allowing psychology to advance into a greater science overall.
Hope this helps.