I generally assume a reliable narrator, unless we have reason to believe them to be unreliable (e.g. Lockhart).

Firstly because as a matter of fact, we know that JKR delivers worldbuilding to the reader via characters, and her intention is absolutely for that worldbuilding to be accepted as fact. Even Bellatrix delivers worldbuilding to the reader.

Secondly because reliable narrators are a prerequisite to having a worthwhile conversation - if the narrator is unreliable, then there are no facts, and if there are no facts then there's no point talking about anything because nothing can be known.

 

I think that’s right. I would just point out that the line about persons vanishing in the present is actually a quote from the fictional professor Saul Croaker, and I wouldn’t describe the vanishing of persons in the present under a scenario 2 as being a falsehood. Anymore then I would describe an article proclaiming Voldemort as the most terrible dark lord as being false; both are subjective claims that are only true from a certain point of view, but clearly neither are falsehoods.

In any case this is besides the point because I agree with you that scenario 1 is probably what J.K. Rowling has in mind.

The important thing to emphasize is that the scenario is consistent, and adequately explains why wizarding folk retain their memories of vanished folk (they actually existed) and how they could have existed despite the actions of the time traveler (the interceding past is causally closed to the time travelers past period). It also doesn’t require us to postulate anybody gaining memories or anything like that.

Nice discussion I must say; I had no idea it would be so fruitful when I started the thread.