Hex,
I think you're over-worrying on the answer you got from Rubbermaid. First, they make coolers, and testing to 120 F is overkill for the intended purpose. FDA extraction testing is not cheap, so why test at temps the product isn't intended to see? Second, if the plastic passed extraction testing at 120, why would it fail at 160? Third, if you think there are any water soluble extratables in the plastic, then soak your cooler in 170 degree water with a little dishwasher detergent (to help wet the plastic) a few times. That will extract anything near the surface. Each time you do this less will come out, and after say 3 times, you can go ahead and use as a mash tun with little fear of extracting anything else out. If there is anything extractable in water.
And by the way, how do you know the mass difference in the one styro cup your friend measured wasn't just experimental error? Do the same experiment with 10 cups and you get closer to a statistically and scientific correct answer, if the weight difference is larger than the uncertainty in the measurement.