I'm afraid I can't really answer the visa question - I hope someone else can. I'm SURE there's a way for her to go as your wife. I would go ahead with the job. As for her working, that's going to be almost impossible, unless she teaches English illegally (very easy to do and should be considered). I've known a lot of American women in Spain because their husbands got a job there, and it drove some of them crazy that they couldn't work. A few - with really good connections at the US embassy - got jobs at the embassy or consulate offices, but don't count on it.
You may want to read through some of the threads on Americans getting work in Spain. If she's a teacher, she may be able to tutor kids who go to the American School of Madrid. The pay for tutors is higher there than in the US. Can she become a British citizen? That would make it all very easy!
Good luck!