Why use citric acid and sodium hexametaphosphate in cheese sauce?
Citric acid and sodium hexametaphosphate are often used in processed cheese as emulsifier. These kinds of salts improve the protein's swelling capacity and emulsification and thus inhibits the leakage of water or fat from the product (forms metal complexes). Some salts are also acid buffers. 1 In this wikipedia article  (in German, but chemical names are quite similiar to the Engish ones) is a list of possible substitutes of citric acid and sodium hexametaphosphate. Or just look here (in English) for E 325, E 326, E 327, E 331, E 332, E 333, E 339, E 340, E 341, E 450, E 451, E 452. 

The receipe of the cheese sauce has the same ingredients of common processed cheese2. 



1  Source: Wikipedia: Schmelzsalz. My bad attempt to translate this article :-\
2 "Processed cheese [...] is a food product made from cheese (and sometimes other, unfermented, dairy by-product ingredients); plus emulsifiers [note from me: the mentioned salts], saturated vegetable oils, extra salt, food colorings, and/or whey [note from me: or watery liquids like water or milk] or sugar. "