Hello Demi,
I'm doing the opposite of you. I'm a mature American planning on living and studying in England
American Universities are very different than the UK . Universities here admit based on many factors, not just academics, and this applies to even elite universities.
It's also 4 years here for a degree. You will be forced to take general education requirements, not just maths
A major is your main subject, and will have a list of requirements. A minor is not required, it's up to you if you wanna have a major and a minor. A minor requires less work than a major. It's not a dual degree
Very hard to get a scholarship as an international undergraduate student. You would need to be a gold medalist at the imo, or something similar.
You also apply to universities without specifying a major. All admits are free to choose whichever major they want, this is normally done after 2 years.
The standard first year maths courses btw might consist of courses you are already very familiar with. The average first year student in the United States has not specialized before university, and therefore might have a broader base of knowledge, but very few would be be able to deal with the maths required for a2 level maths and further maths.
The way maths is taught in the United States at the secondary level is different than pretty much everywhere else in the world. The textbooks tend to be awful, and discovery based. You have 4 years at what we call high school
Maths is not taught as an integrated subject , which undoubtedly contributes to our poor problem solving abilities. When we come across a problem that isn't pure algebra, or pure geometry, etc we don't know how to handle it.
I'll give a standard 4 year high school maths sequence
1st year Algebra 1
2nd year- Geometry
3rd year-Algebra 2 with Trigonometry,which can also be called Precalculus
4th year- Precalculus or Calculus
We do have AP tests, which like the UK, are exam based, but the only maths based AP exams are Calculus or Statistics. They are only 1 year courses, and you only take 1 exam
You also don't have conditional offers. You take your exams, and the scores are known before you apply to universities.
I greatly prefer the UK system. You know what you need to do in order to be an acceptable candidate. There's a lot of variables here which have nothing to do with your academic record
Your scores will definitely be recognized by universities here. MIT is known for maths, but so are Harvard, Princeton, Caltech,etc
We have an annual examination for American and Canadian undergraduates called the Putnam. Harvard has won it the most times , by far, and tiny Caltech is second
http://en.wikipedia.org/wiki/William_Lowell_Putnam_Mathematical_Competition