a helmet with an uncertain history is useless
Sorry for being so blunt, but this is just all nonsense. The 'point of evaluating a used helmet' is that you can't. You could of course decide to ride without a helmet, but once you go through all that effort of buying and wearing one, it would kinda majorly suck if it didn't do its job when you need it.
Get a new one from a trustworthy source (i.e. one where it didn't sit for a year next to the solvents and paints, or in the window). No need to spend 500 bucks, just spend 200 or 300 on one that is comfy and fits, and be done with it.