It sounds like you've done a lot of things right with this phone. The only thing I would add is to search under the microscope for the specific components that look oxidized. Try adding some flux and trying to reflow these components, or just replace them from a donor iphone board. I have seen a solution for temperature sensor troubleshooting on the iPad 2, so it is likely hardware. If you're still stuck, the last thing to do is just sit with the schematic, and try to see a connection between the areas of the phone that were most damaged by corrosion and anything that seems like it could have a role in temperature sensing.