I had a typo earlier - I said it was in 11 where you find out about Morgana's background, but I meant to say 12. That's the ep that's all about Morgana and you learn about her father and how she came to be in Camelot (which is so far from the legends that it breaks my brain, but whatever).

As for their accents, I really hate to say it's a "British" accent because there are so many of those, and they're not going for Received Pronunciation (i.e., Queen's English) either. Basically, they just seem to be using a bit more enunciated versions of their natural accents (save Colin, of course, who's putting on an English accent, but I could not tell you what region -- my ear's attuned to Irish accents, not British ones). But Katie has the same accent that I used to when I lived in Dublin.

I think Arthur's confusion over Morgana is that he's been living with this girl who he's thought of as his sister for years, and now that they're growing up, all of a sudden he sees her as a sexual creature. That doesn't seem that extraordinary to me. It seems they've both grown up dismissing/teasing each other. Now they're starting to do that in the sexual arena (Morgana more actively, it seems, with her revealing dresses and all, which makes sense since girls mature earlier).

I think you're right about the practical applications of magic. I also think that now that the enemies he's facing are magical, he needs to study to understand/defeat them. His magic is different, as Gaius points out, but elemental magic might be too unfocused to enchant a sword to slay an griffon or to transform a stone dog.