Makes no sense...
If you say your water conditions are spot on, then there should be no reason why the mandarin would have died. He only arrived from ORA within a few days, and he may very well be still stunned by being in a bag for a good amount of time. It took four days to see my first spotted green mandarin eat anything from the day I first got him. It took a week for my blue mandarin to actually start acting like he was interested in putting anything in his mouth when I first got him.
Your goby could have done the carpet surfing suicde...do you have a glass lid on your tank? There could be some aggression too behind the scene. You may find your mandarin when you break your tank down, if you so choose. An option, if you decide to stay on, is to clear out some of the live rocks and only keep ones that are simple in form so that there isnt all the nooks for the mandarin to hide. I have live rocks too, but I position them so that I can easily find any fish at any time. You can do the same, given your tank is only a bit bigger than mine. By doing this, you can have greater control of the whole situation, and mandarins need their people to be in control. Dont give up so soon. mike