Wow, sad story.... My sympathies....
I am reminded (as should happen more often); "there but for the grace of god go I..."
I'm sure that things differ greatly from country to country, so I really wish I had an answer which was more helpful, but here are some thoughts;
The aircraft must have had a "purpose" for the flight, and therefore a flight authority (permit) issued by the national authority to allow the flight. Any flight authority for a test flight I've ever seen has an associated application, a part of which would contain an attestation that the aircraft is "safe for flight" made by a qualified signatory representing the builder or maintainer of the aircraft. If the crash was found by investigation to be the result of something, particuarly something the pilot would not detect, being not safe for flight, I thing there is some basis for a lawsuit there. The national authority would be the ones to ask if there are any requirements for the insurance you speak of. If such a requirement exists, they would know....
I am not aware of any requirements for insurance of the kind to which you are referring. The aircraft probably would have been insured for third party liability (so the bank was covered), but that would not help you. If such insurance were in place, you would have known, as the pilot would have had to have been accepted by the insurer to act in that capacity. As for "manufacturer's" insurance, I am certain that there is no requirement to be carrying what you are thinking of - Many companies just could not get it - unquanitifiable risk. I had a friend in an extremely similar situation, I think that he did not suffer, the company (Seawind) does not seem to have survived his mysterious crash either, from all outward appearances.
I keep myself well life insured, knowing that nobody else may be covering me while I'm test flying.
Good luck, and best wishes...
Pilot DAR