Once again, everything looks pretty good on the translations!
I wonder if this is simply the explanatory の. If so, then rather the question of “Did she not understand anything at all?”, it would be more along the lines of, “Is it that she didn’t understand anything at all?”
The logic here is that it’s taking a sentence (「こいつ何にもわかってなかった」), and turning it into a “It is [noun]” sentence. The の turns the sentence into a the “[noun]” portion of “it is [noun]”. And since it’s a question, in English we’d swap the “it is” around to “is it [noun]?”