The silver-thiosulfate complexes are absorbed into the paper fibers and not the silver grains. The washing problem is not associated with the emulsion but with the paper backing. This is why film and RC papers require less washing time than FB papers.
As was mentioned previously many salts can be used as hypo clearing agents. What is involved is a compeetition for adsoption in the paper between the clearing agent and the thiosulfate ions. By making the clearing agent ions more populous than the thiosulfate ones insures that the thiosulfate ions will be desorbed. There is nothing particularly special about sodium sulfite other than it is somewhat more effective.
Determining the exact amount of silver in a print would be very expensive. One method would be neutron activation analysis. Classical quantitative methods would not be accurate because of very small amount of silver present.
There seems to be some confusion as to what acceptable permenance involves. There are two considerations. First is the reduction of silver and second is the reduction of thiosulfate to an acceptable level. The amount of residual silver in a print is determined by the amount of silver in the last fixing bath and the effectiveness of the wash. The amount of thiosulfate is determined only by the effectiveness of the washing of the print.
The amount of thiosulfate present in a print can be roughly determined by using Kodak HT-1a solution. This involves the amount of decolorization of a potassium permanganate solution. The more thiosulfate the more the test solution is decolorized.