Tror ni verkligen, Dumbledore, att ni kan förklara altsammans i ett brev?
An excellent question, Professor McGonagall.
I started reading Harry Potter in translation (och Den Vises Sten) for practice, and I've noticed some characters using "Ni" as a singular pronoun.
For my purposes, I'm happy with the rule that Ni is only ever for the plural, but I'm curious why the translator chose to use it. Does it sound old-fashioned and wizardly? (The extremely mundane Mr Dursley doesn't use it). Is it because the character is talking to someone very senior and respected? Could it be because the character speaking is Scottish, and the translator chose to reflect that by using a finlandssvensk turn of phrase? Or does it just sound like a flawed translation?
Since in English, people use a lot of forms of address that have a much higher level of politeness/lower egalitarianism than we would normally use in Swedish (for instance calling people Mr or Professor), some translators feel they want to try to mirror that somehow, and ni is one option.
It's actually a quite new feature in the Swedish language, to use "ni" like in French or German. Before "du-reformen" Swedes used titles (herr, doktorn etc.) to people of higher rank, and "ni" to people of lower rank. So older people often get offended when young people call them "ni".
I think you are basically right in that the purpose might be to make it sound more old-fashioned and wizardly. It makes it sound like the book is from the 50s or earlier. It might also be to increase the social distance between the people. I have a vague feeling, based on Veros comment, that it is used much more to that effect in fictional writing than in real life.