But wouldn't "tu" be viewed as extremely rude in that situation?
Well, in the French version it kinda went with the fact that Nero calls Pike "Christopher" instead of "Captain Pike". I've seen it as Nero wanting to show he has no respect or fear of the Federation and Starfleet and doesn't recognise its authority.
I would have thought perhaps Nero started out with "Vous" but then perhaps somewhere in the interrogation scene switching over to "tu".
Frankly I can't remember if there's a pronoun switch or not, 4 viewings aren't enough
Spock Prime does use "tu" when talking to nuSpock
Is that from dubbing or subtitling?
That's from dubbing, alas! I've been unable to see the movie in subtitled OV so far, no theater in my area shows it that way.
I can't say for Voyager, I've only seen the pilot. I'd have to buy the DVDs to see it and frankly, I'm a bit broken right now

I'm lazy so I always watch ST (and anything in English) with subtitles, but with the English ones. The French subtitles make me want to throw something at my TV too often
Anyway, I don't like the existence of "tu" and "vous" in French, I feel it complicates things. Sometimes you use "tu" by accident when you shouldn't have and it can lead to awkward situations

When you have enough practice in English, you're able to feel if the person talking to you is using "you" as "tu" or "vous" by her tone of voice.
I've always felt that throughout the years, Spock, Kirk and McCoy kept on using you as "vous", despite their friendship. I don't know, it seems to me that there's some big respect thing going one, no matter what.