More than likely, these BG are CNBG, I'm guessing..
I'm highly suspicious that a fast "rate of change" is more than likely the culprit here. The rate of temperature change is more of a factor than the actual temp.
I keyed on one other item you mentioned.. "algae on the fish"... are you sure this isn't fungus that has taken on an "algal" color?
I'm personally leaning in a direction of a stress kill more than anything else. Not saying low DO wasn't an issue, maybe a contributing factor but you went through, as you said, "the worst conditions in 100 years"..
My thoughts as well. CNBG will eat aggressively in 40 degree water, but even with warmer water, they will shut down if there's a rapid water temp drop. I haven't found any post ice over CNBG morts, but there are some larger ones that have fungus on them. I haven't seen the fungus on smaller CNBG.
