> the executioner knocks on the prisoner's door at noon on Wednesday — which, despite all the above, was an utter surprise to him.

Then it follows there are two false epistemological claims in the text. First by the judge:

>He will not know the day of the hanging until the executioner knocks on his cell door at noon that day.

This is false, as you correctly reason, since a knock on Friday would come as no surprise (at the time it occurred).

The second false claim, this time by the narrator, reinforces the first:

>Everything the judge said came true.

Not every claim the judge asserted was tested by the recounted physical events: There was no knock on Friday.

The judge says the prisoner would be surprised exactly when the knock on the door happens (not before). As a consequence, the prisoner is right to believe he won't be hanged at all (following his reasoning), but as a consequence of that, he is surprised when the knock happens. In turn, as a consequence of that, the judge is right. Therefore, both the prisoner, and the judge are correct. Thus the paradox.

Incorrect. Not getting hanged is also a solution, which doesn't allow induction.