Do toasted nuts quickly lose flavor?
Toasted nuts keep their flavor pretty well, the way to keep them is the same way you'd buy them at the store, ie a sealed container. 

What you get from freshly toasting nuts is more aromatics than flavors. Most of the aromatics go pretty quick so there's no preserving them, it's use 'em or lose 'em. Could you substitute pre-toasted nuts? Sure, it won't be quite the same, but it will be close.