“Snake showed Fudge incontrovertible proof in the form of his Dark Mark, and Fudge just shook his head.”
That’s not quite as incontrovertible, unless you accept my memory tampering answer.
There are not varying degrees of ‘incontrovertible’. There was no other plausible explanation for the Dark Mark, except Voldermort’s resurrection.
And I do accept your memory tampering answer, inasmuch as whatever the facts of the matter, Fudge would have, at that point in time,completely ignored it. Had Voldermort himself walked into the Hospital Wing at that point he would have probably found an excuse (when that scenario actually did occur, he wasn’t the only witness, and he had a year’s worth of deaths to account for).