By the way, about Arthur Blackthorn marrying a faerie, can you imagine how Valentine would've reacted to that?
Well, to answer the first: My Blackthorn characters have been around for quite a bit longer than the Shadowhunter Blackthorns...and they are certainly no angels. 
And I could see Valentine being more than a little bigoted toward them...but at least they didn't have Demon Pox ^^
I should've known, just from reading Scarlett's bio, that they weren't Nephilim. First of all, an iratze could've easily fixed Scarlett's eyes. Second of all, the bio said that Gabriel killed their father, and as Jace once said, "A Shadowhunter who kills another of his brothers is worse than a demon and should be put down like one."
But, you didn't answer my other question: Did you see the movie?
Oh, I missed that question, sorry. No, I've not seen it yet. 
I am waiting for the dvd to come out (I live too far from any theater complex).
Not necessarily bad...just...I dunno...I enjoy the reading of books and there always seems to be something lacking in the transition from print to film. I think it comes down to the concept that reality can't stand up to the fantasy.
You get introduced to the literature. You read the novels, and in your head you build ideal mental representations of the characters. How they look, how they talk, how they move. Then when the movie comes out, the actors/actresses, directors etc...have their own interpretation that may conflict with your own.
I can understand the need/want to take the film in a slightly different direction than the books so long as much of the core plot remains in place. It breathes a sort of new life into the series, keeping things fresh but familiar for the target audiences.