Replacing Headphone socket FP1U

Try this one instead: http://shop.fairphone.com/spareparts/motherboard-1.html

By the way: the FP1 motherboard will likely not be produced any more, as the FP1U motherboard works in the FP1 (but, as you mention, not the other way around), see here:

Other than that, you could have a chat with a local repair shop - if they have soldering pros they may want to try - though likely at your own risk. The topics you mention may give them a better idea of whether they will be able to manage.