Bumping the thread to submit a few more personages:
Alexi: Mads Mikkelsenpic 1pic 2
Probably best known for playing Le Chiffre in Casino Royale, he's Danish and IMO possesses the cold, sterile beauty necessary for Alexi. And before people start harping on about "blah blah not blond enough shreik shreik" I'll remind you that we're theorising about these books being made into major motion pictures of which large sums of money are generally set aside for such amenities as wigs.
Madame Vega: Cate Blanchettpic
Surely needs no introduction. I'm aware that I said in my first post that I intended to eschew using major names however I think for a character like Vega a very famous actress would be appropriate. Also to look at it from a producers perspective, should Obernewtyn be filmed it would need at least one marketable name attached to the project for fear of box office failure considering the wider world has never heard of these books.
Alternate Madam Vega: Tilda Swintonpic
Same deal as with Cate; I think a role like Vega is best portrayed by a well known actress and both of these women have the grace, beauty and sophistication necessary, not to mention the talent.