is lurking in the shadows and cackling
over here Now, this is where it starts getting really fun ^_^
I'm sure most of you realise that I'm an Dumbledore/Grindelwald shipper as well. I wrote one of the earliest fics for the pairing, but then wandered off away from fandom and never explored the relationship any further (there's still a WW1 piece I have sitting in my imagination, just waiting to make it onto the page). Once I started thinking about the idea of Hogwarts being sealed and Dumbledore being somehow ambushed in his own office, I thought of including Grindelwald as well. It's part of my personal canon that Dumbledore visits Grindelwald quite often (he brings socks every Christmas, and is always a little sad when they're not appreciated). If Dumbledore got ambushed on his way back and the Floo stayed open, well, yes, who wouldn't take that opportunity? (Mind you I think Grindelwald's probably been capable of escaping for years by this point - evil genius in his own stronghold with nothing to do - he'd figure it out).
De-aging them and giving Dumbledore amnesia was just for the pure fun of it (the others aren't changed because they're already seventeen). Grindelwald adds a whole new element of danger to this - although he seems to be penitent at this point, and is certainly not impressed by this upstart Voldemort, he's still the Dark Wizard Grindelwald, Evil Wizard Mark One.
As for the pseudonyms, Abbé Faria is from the Conte of Monte Cristo, where he is the protagonist's cellmate, who instructs him in philosophy and statecraft and so on. The real Faria was a pioneer in the study of hypnotism, with revolutionary leanings. As far as Napoleon goes, I think Grindelwald quite likes to fancy himself as the wizarding world's Napoleon, with Nurmengard his Elba (or ultimately, his St Helena). Hesperus is the Greek name of the morning star, aka, Lucifer, fallen angel. Call me Ishmael is the opening line of Moby-Dick, and Ishmael is the eldest son of Abraham, a symbol of exiles and outcasts, albeit one who is ultimately repentant. Yeah, the fact that he's had nothing to do since 1945 but read anything Dumbledore happens to send him is going to make him fun to write. ^_^
I'm still trying to get a sense of young!Albus, but I read him as being very earnest, kind and brilliant, but more than a little unworldly. SG-1 watchers, think early season Daniel Jackson, complete with stubborn streak.