In the pre hardener the sulfate and carbonates are inorganics. They are stable. The formalin inherently resists organic degradation. Yes, the pre hardener is reusable; it may discolour as any film overcoat partially dissolves in it as it acts; I would not worry about that.
Keep in a well sealed nearly full bottle to keep the gas in the solution. I'm still at about 970ml of a litre of formalin (saturated solution of 37% formaldehyde) I bought over 5years ago. Once I get to where there ia a larger gas space in the bottle, I may elect to add marbles to decrease the volume of air for the formadehyde to off gas into.