Can there be more than one combination of reactant/product concentration for a specific equilibrium constant?
A single "elementary reaction" does not involve an equilibrium.  

There must be a forward reaction and a reverse reaction to have an equilibrium.  This is at least two elementary reactions.  

Considering an equilbrium reaction: Just as a square encloses the most area for a rectangle of given perimeter, if the sum [A] + [B] is kept constant,  [A][B] is maximal when [A] = [B].  Deviation from this maximum will shift the equilibrium away from the product and toward the reactants.  In other words, if the system is at equilibrium with [A] = [B], and [A] is decreased and [B] increased, keeping [A] + [B] constant, there will be net reverse reaction until equilibrium is again reached, and [AB] at equilibrium will be less than orginally.