There is now way they can say that hoses or a dump valve would cause this.
These are standard items that have been upgraded and no way can that be attributed to an engine fire. You must be able to get a lawyer to compile a case for you. Perhaps even the threat of this action might be enough to make them see sense.
It might be worth looking into getting an expert engineer to compile a report on the reason why your car caught fire for legals to use.
Hope you sort it, I bought an extinguisher after reading of your incident.