My dog went in for an MRI and left with brain damage. Insurance won’t pay and hospital is demanding payment

Hi all, i have a few questions and I could really use some advice. In Scotland.

My dog received an MRI for a pre-existing condition with her spine last year. She had an unforeseen adverse drug reaction to an emergency drug given when her heart rate dropped under anesthesia and had to be kept in the ICU for an extra 7 days which tripled the bill. She left the hospital blind, with seizures, could not walk, eat or pee on her own, and had severe brain damage that slowly improved over 6 months until she passed from an (unrelated) lung cancer. I cannot be certain, but I am sure if there was neligence by the neurologist that caused this. I did not think so, as the notes sent to my regular vet say there was an unforeseen drug reaction and nothing else was noted (that I saw) regarding something going wrong (ie overdose, wrong drug, not monitored anesthesia etc.). Our regular vet received the notes from them and from what she told me it seems my dog just had a rare adverse drug reaction, completely unexpected. I work in medicine so I know this happens sometimes. When speaking with the neurologist, they could not determine why this happened, and told me it just happens sometimes and she should improve over time. I didn't think there was any negligence, because wouldnt he have had to tell me if something had gone wrong? I have not fully investigated the procedure myself yet due to how traumatising it has been for me, but am wondering if I should as my dog was severely brain damaged? If I did so, would this have any effect on the insurance/billing issue below?

The second issue is around the billing. The neurologist said the two conditions (the pre existing back issue which the MRI was for, and the adverse drug reaction) are not medically linked as the drug reaction was a rare accident, yet the hospital sent the claim to the insurance all under one claim/condition. Insurance refused to pay this initially linking the emergency to the pre existing condition. The neurologist then wrote a letter to insurance saying the conditions are not linked, one was an unforeseen accident, and they should be considered separate, but insurance said their own vet said they are connected, and because the claim was submitted under one condition, the hospital must think this too. The hospital has now refused to resubmit the claims as two separate claims, saying the insurance already rejected it so it's too late. We have been through the entire complaints process with insurance regarding the neurologists letter and they recently sent me their final rejection. I asked the hospital if their own neurologist said it is two conditions, why were they submitted as one? The hospital told me the neurologist corrected this by sending the letter to insurance. However, the hospital did not actually resubmit the claims to the insurance as separate conditions. They are now demanding payment for the accident/second condition.

Should I ask the hospital to forgive the cost of the accident as a good will gesture? Do they have a duty to resubmit the claims to the insurance? Should I go to the financial ombudsman? Should I get a lawyer? Should I investigate for negligence? What can I do? Thanks everyone, this is incredibly stressful.