I am reviewing your question to see if I can help you.
You have had exhausting situations happen back to back.
First let me say, I am sorry. I am so sorry you have had to be put though so much, I know that NO one can break our hearts like our own babies.
You are doing everything correctly and yes there is hope. I am so glad to hear that you can get him to go in for testing and he does not completely revolt. This means that you have a very hopeful situation because deep down he wants to get better.
There is nothing you can do about the girlfriend, except hope the phase passes, you can not allow her into your home but he will find a way.
My advice to you, would be, pick your battles carefully until next weeks test comes back.
I do advise that you follow through with all personality testing to rule out borderline personality, bipolar, chronic depression etc.
Also let me say that as a mom, you need to know, you have done nothing wrong and you are not alone.
All over the world people are going through the same exact thing. People also tend to not discipline when one parent is lacking, that goes both ways.
Your job right now is to make sure he is safe, doesnt hurt anyone else and you continue to get him all the help that you can.
The fact that you have gone through so much already to help him shows that you are trying with all of your heart.
Also, do not let him have the power of saying he does not have to listen to you because he will just leave.
The truth is, most children that try this, realize once you call their bluff..they are not going anywhere.
Why would he? You keep a roof over his head, food in his stomach and he is in no way ready for real life.
I would also suggest that you communicate with him when he is calm. Do not attack the girlfriend because that will only push him towards her more and give him a reason to lash out. Actually, seem supportive about what matters to him.
Wait until he is tired and calm. Sit on his bad and talk to him. Do not get onto him or start any confrontation.
He is testing you and not being respectful, you must demand that, in a calm way.
One way to gain it is to start to give little freedoms and give value to what matters to him.
Even though you do not like the girl, it is obvious that she has some hold over him. Sometimes when boys his age become active the object of that becomes very important. He will fight you on her everytime. Luckily, she will go. But, the most important thing is to continue to bring him to therapy, get him tested and listen to the professionals.
Do not use your family doctor for meds, always keep him in counseling, so that someone is regulating his meds. Try to find a strong male counselor or psy, someone he will look up too...that can give you support to make him take the medicine.
Most of all. TAKE care of yourself. you have so much going on....and with your RN too..Which is so difficult. Try to keep everything in your life in sections.
One thing you can do is not allowed the opposite sex in their rooms, really as the parent, you can do anything you want to do...its your home. Call his bluff. Stay calm.
Always stay calm. And hopefully your next bout of tests will bring you some answers, treatment and support.
I hope this helps. Let me know if you have anymore questions.
I do want you to know that I am the mother of a special needs 15 year old boy, so I have much insight into this process. I am here to help, whenever you need it. Peace to you and your family.