Your experience with being broken into is describing one of my worst nightmares. I don't know if his previous injury could have left him more susceptible to an injury from rough playing or play fighting or fighting. He was fine for months after he healed from being shot, but then he began having these problems, and it makes sense the playing might be involved, and possibly the past history of injury. I would really think your observation about playing with the bigger dog and being pinned down might be an important fact in this. Did you tell the vet about that, and did the vet listen to you
You shouldn't blame yourself, because it is totally understandable that you would want another big dog after your experience, and you had no reason to keep them separate because he seemed fine. Teenaged dogs (and cats) can be full of energy and not know when to stop.
I really think if it was my dog, I would absolutely want him on some kind of anti-inflammatory (like prednisone) to see if you can get any improvement. Also, if it was my dog I would ask about a muscle relaxer. If you've ever had a sore neck, the first thing you want is a muscle relaxer. Ask them "What could it hurt???"
You said they saw something compressing his neck in one image but then they shrugged it off and decided it was nothing. I wonder if doing additional imaging focusing on that same spot might be helpful, so you can take a second look and be more sure about it.Don't take 'no' for an answer.
They don't understand how important this is to you and your family, to help this dog.
Right now while he is lying down, do you have malewraps for him to keep him dry? A malewrap is also called a belly band or a male doggy diaper. It's a wide belt that goes around the waist and fastens with velcro. You put an absorbent pad (like Poise or another adult incontinence pad) in it and change as needed. It is extremely important to keep his skin dry while he is laid up. If he lies in wet bedding, he can quickly get a urine burn on his hip and they are easier to prevent than to treat.
If he still has a good appetite, that is fantastic. He's 4 years old, young and strong. I think you need to really go to bat for him. Tell them he appears worse because now he can't walk at all, and furthermore he seems painful, and you know
this isn't how an FCE goes.
Actually, if you aren't getting anywhere, something else you could try is to ask elsewhere. You did what anyone would do if they had the money. You went to a specialist. But if you are getting totally nowhere with the specialist, and if you know another vet who will at least listen to you, talk to them. Tell them you did the tests, they said they couldn't find anything, they decided to call it FCE, but you know that what you are seeing does not match FCE and your dog needs help. Maybe your regular vet would consider treating him based on symptoms. Or maybe he will know of another place to refer him to.
I have had 2 unfortunate experiences with imaging here. In the first case they missed the problem completely. In the other case they weren't sure, and based on my previous bad experience I wasn't going to trust them, so I decided to have my pet treated by our regular vet and that turned out to be a good decision.
The other thing you might do is, ask your regular vet if s/he knows whether the equipment in use at the specialty place you went to is top notch. Here where I am there is a different specialty practice 90 minutes away that has the latest and best equipment for imaging. I've decided next time I need imaging, it will be worth the drive. So perhaps your regular vet knows of another practice within driving distance that happens to have newer equipment that might give better results.
I am not a vet and I have no idea what kind of equipment or expertise you have available at the place you already took him to or anywhere else. I'm just trying to think of alternatives since you still don't have the information and treatment you need. You and your dog deserve to get to the bottom of this!