@stefano
I read your post, and stuff like this is incredibly infuriating. You did everything right, someone made a tiny mistake and the insurance company is exploiting it to the fullest extent possible. And you are essentially powerless, unless like you said you get an attourney and battle your insurance in court for a couple of years. And who has the time and energy for that...
I had my bike stolen once, which was insured, and they also weasled them out of their obligation to pay because of a technicality. So I bought a new bike and canceled the insurance. No point in having one if they're not paying...