A vet does not have to xray a knee in order to grade the degree of LP. Since you are in Houston, I highly recommend that you seek a second opinion at Gulf Coast Veterinary Services. Dr. Brian Beale is a surgeon I have used many times. He would be happy to evaluate her and would not recommend surgery unless it was definitely necessary. I do not believe it is true that you should wait until she is finished growing.
Here is an article that he wrote that might help you in terms of understanding the grade:
http://www.yorkiehavenrescue.com/patellarluxation.html