Admissions is getting better and better at choosing applicants who are most likely to choose those majors - think people who took a lot of science and math, and people who went to "technical" high schools and whatnot.
To your other questions: About 33% of each class is from some form of prep school or college, though I suspect those numbers will go down with time. I do not think they are a good predictor of future success. My plebe roommate was home-schooled and graduated among the top of my class.
Courseloads are heavy. I was a non-STEM major in Econ, but my B.S. of Econ is significantly different than a B.A. in one. My B.S. required that we did correlation / regression analysis (think proving correlation is or is not causation). When I talk to a B.A. Econ major, half them say "yeah I've heard of that," and the other half look at me cross-eyed. So even within my major, there was a heavy emphasis on math, despite it not being a "STEM" major.
Life at the Academy is busy, but structured. My other options were Penn State and Villanova. I think because there was less of a social life, and there was such a culture of excellence at the Academy, I was driven to work harder (i.e. being good at school and being in great shape are both highly valued among your peers at the Academy) and given the resources to do so - and I think my grades from college were better at USNA than they would have been at Penn State or Villanova where I think I would have partied a little too hard. I can genuinely say I appreciated the regimented lifestyle in terms of being a resource to get the grades I earned.