Vitamin C is used in IV due to its antioxidant and protective properties, not because it is a mercury chelator. The distinction is important.
The University of Michigan information you posted said nothing about mercury chelation with Vitamin C. It was a general page defining chelation, and using a definition that is more broad than the one we use here.
Btw, the Onibasu link shows that they also tested for fecal elimination of mercury and could find no effects from the Vitamin C.