At this point you will need a good local workers' comp lawyer to help you manage this legal problem. I think the legal problem is fixable but you will need a good lawyer to help you fix it. That in turn will result in you getting the medical care you need. But it is a mixed medical - legal problem.
This answer is intended as general information and not as specific legal advice. If you want to have a free consultation with me, please contact me through AVVO.
You certainly do have a case and this is exactly the sort of problem where it will be useful to have help from an attorney. The situation you've described is not unusual and based on the facts as you've stated them, the claims manager should have approved your doctor's request. To iron out this problem, communication with L&I is necessary to get the request approved, or to get something in writing (an order) that you can bring to a judge at the Board of Industrial Insurance Appeals and have a hearing on the matter.
I also think it would be a really good idea for you to see a workers compensation atty and sooner rather than later. The Dr identified other conditions that are significant and the insurance company will deny that care up to the point that a judge tells them that they have to cover it.. If you came to me with this scenario in my state I would file for a hearing almost right away because I have seen this many times in the past.
Your questions need to be addressed by a competent local workers' compensation attorney as there are some pertinent details which may affect the answer. Most of us offer free initial consultations.
Some basic concepts:
You are entitled to treatment by a doctor of your choice for conditions allowed by the Department or self-insured employer. Your doctor has to have signed a contract with the Department of L&I.
The Department or self-insured employer has the right to contest newly contended conditions until you prove that your industrial injury or occupational disease was a proximate cause of these newly contended conditions.
Generally speaking, you do not have a right to a second opinion under your claim. However, your attending physician does have a right to a second opinion. Perhaps your first doctor felt slighted or offended by your seeking the opinion of another doctor. For whatever reason, that doctor has quit. At this time you need to select a new attending physician for your shoulder.
Plesae call a competent local workers' compensation attorney for specific advice. Good luck to you.