That is absolutely fascinating to read about! It's too bad that there really isn't a good way to convey that kind of nuance in English translation. 'Sarcastic politeness' seems very Erik to me, especially given the performances of certain ALW Phantoms like Hugh Panaro. The shift from vous to tu in the 'your soul is a beautiful thing to give' exchange maybe makes me think that Erik is slipping up momentarily, not quite maintaining complete control over his Angel act and reflecting his desire to be close to Christine in the way he addresses her. That might be why he also says 'my child' right after that- like 'oops I slipped up with tu- got to say something to reinforce that I'm supposed to be this heavenly authority figure who's formal and above her'.

Lately, I've been reading quite a bit of My Hero Academia fanfiction (I assume it shows up in other anime fanworks as well, but frankly MHA is the only anime I've ever watched), so what it immediately reminds me of is the sort of subtle displays of relationship and changing dynamics between characters in what honorific suffixes they use when referring to each other by name, like -san, -kun, -chan, etc.