Proof for

Proof for
 

p.739
-then we'll have your body as proof for the next idiot who didn't believe us....

Could I use of without change of meaning?

"For" shows that "the next idiot" is intended to see the proof. Using "of" would mean that "the next idiot" is the owner of that proof.

Your answer is wrong; if FOR meant 'intended for', Hermione wouldn't use 'didn't'.

Is there anyone else dead before the person they're talking to?

I agree with AdA that "for" and "of" are NOT interchangeable without changing the meaning of the original text.

This sentence appears to be the second half of an "if X, then Y" conditional, or hypothetical statement.

"Then we'll have your body as proof for the next idiot who didn't believe us..." = "Then we'll have your body as evidence to present to the next idiot who didn't believe us." That is, the body is present as evidence to a person who is identified as "the next person who didn't believe us".

"Then we'll have your body as proof of the next idiot who didn't believe us..." = "Then we'll have your body as evidence about the next idiot who didn't believe us...". That is, the body is evidence about the next idiot; the sentence says nothing about what use is made of this evidence or to whom the evidence is presented.

Good answer. Thank you, but I still haven't figured the answer to this: why 'didn't'? If Hermione is talking about a 'next' idiot, then this idiot is a future thing, then it should be 'don't'. Of course sometimes we could use the past tense to talk about an action viewed from the point of the future:

-Tomorrow John will kill Peter. The day after tomorrow Mary will find this out. Mary will hate the one who killed Peter. (now, Peter is still alive; the act of killing has not taken place)

But I think this is not the reason to explain why Hermione uses 'didn't'.

Using the past tense "didn't" is either because the action is being revisited from a point in the future or because the action is being cast in the subjunctive mood.

In the indicative mood, "doesn't" (not don't) makes sense. But if the viewpoint shifts to a future event and looking back, or if there is doubt that the event will occur, using "didn't" makes sense.

Excellent answer. I like your use of revisit.