Hi and welcome,
I would strongly suggest you consider seeing a neurologist that specialises in MS for a second opinion!
I honestly can't tell you very much about 'defuse sclerosis', from my understanding its quite rare to start with but it's more commonly associated with children than with adults. The potential for a misdiagnoses is likely high because the diagnostics evidence is very similar to MS, but there are a few specific details that help make the diagnosis of defuse sclerosis more clear instead of MS or any of the other related differentials.
It often mimics intracranial neoplasm or abscess.
Viral encephalitis or viral meningitis, subacute sclerosing panencephalitis (SSPE), progressive rubella panencephalitis, brucellosis.
Primary CNS vasculitis.
The information is often intermingled with it's twin conditions like schilders disease, below is the easiest to read and understand article i've been able to find but keep in mind when something is rarely seen in adults, most of the information you'll read about will be in relation to the more common paediatric patients which isn't typically the same for adults, try not to read too much in to what it might mean for your adult situation.... https://emedicine.medscape.com/article/1145927-clinical
I can't tell you if there are any support groups in your country or near your area but you might try contacting your closest MS society because they will likely have some information or direction for you.
I'm at a complete loss with what this diagnoses has to do with a car accident, i'm in oz and we don't have the insurance situation that i often only read about.....I do recall a couple of MSers in the past who have had their insurance denying access to MS treatment options and either the diagnosing neurologist took on their insurance company or a specialist lawyer got involved.
All i can think of that might help is to contact your diagnosing neurologist and specifically asking him/her what this diagnoses actually means for 'you' specifically and if there are any specific medications you should be taking and if so, what help or advice do they have in dealing with your insurance company.
I hope that helps.......JJ