As for distinguishing between tarnish-free silver and nickel silver, copper, pewter or stainless steel, that I cannot do either in a blind test by smell alone and doubt the ability of even a trained nose.
I have done precisely this in single-blind tests with my brother; rubbing the thumb firmly on a piece of silver accentuates the smell, and unless you're testing pristine bullion or immediately polished silver, there is almost always some tarnish to detect.
I have to negate your argument that:
If smell is used in conjunction with other techniques, but can only provide marginal corroboration, then it becomes nothing more than a reinforcement of the tester's existing opinion and therefore worse than useless information.
If I test an object and it does not smell like silver, then I know that it is not silver regardless of my original opinion. I have refrained from buying unhallmarked or curiously marked spoons after they smelt neutral as indeed I have bought similar spoons which did smell. How on earth can that be useless information?