How can I sand the underside of an enamel sink?
If you could sand on the bottom of the sink - by the time you removed 1/2 inch you would be way through the sink thickness. So I think that the sanding of the sink is a non starter.

Since it was the counter top guys that screwed this up you should demand that they be the ones to take the steps to correct this problem. If that means ripping out the counter top to install a new one then that is a cost that they will have to bear.