In regards to the systems with the tanks that feed the windshield washing squeegees and at some locations for sale, I'd not bother at -20F fluid.
I can't recall for certain, but at least one of the stations nearby that uses this system perpetually has slushy squegees anytime the temp is below around 5 or so... perhaps the fluid isn't the -20 stuff, but who knows.
Second, practice here I've had much better luck with the fluids rated at -35F or so in our climate. I've found the -20 stuff to often freeze in some of the smaller washer hoses or nozzles at warmer temps. Perhaps the Methanol evaporated, but regardless... Your mileage may vary, but since I swtiched to the -35 stuff, I've had very, very few winter performance issues.
Last, on the legionnaire's disease, its an interesting pathway. A quick google search shows a flurry of articles and information around 2010 and 2014, but not a lot else. Many of the articles noted that while it was possible, there was not enough information yet to suggest that it was a common pathway to people contracting it. More research was needed...