Just keep pushing Nokia. As stated earlier, be polite and explain you haven't done anything to it. Make them tell you what could've happened and tell them it didn't. There is no reason for you to not get it repaired, if it's not your fault. Just don't back down cause the first guy tells you it's not their problem.

I don't think the usb will ever fail. I've never understood how people have had problems with it? Probably stabbing it in the wrong way around or somming :/