Check the local warranty conditions, you may yet be able to get an RMA.
Also try a nice, friendly E-mail to their support people, they may be understanding-nothing ventured, nothing gained.
At the risk of incurring the wrath of the rest of the community I'll also suggest you try posting on other forums as well, after all, the more people involved, the more likely it is you'll get a successful resolution of the problem. Techpowerup and Guru3d also seem to have their share of gurus keen to help fellow PC users.
I don't think it's possible to replace the sensor and a BIOS edit needs some very specialised knowledge the only other option I can think of is a watercooling loop to drag the temperatures down below the shutdown point, but that may very well be more expensive than a replacement card.