That is an excellent question. Personally I did not manage to what we call validate many classes. I had some courses that would have worked at Texas schools but definitely not here. My roommate on the other hand validated at least 3 classes and is doing well. I truly think that USMA is pretty good about putting you in classes that fit you well, permitting you speak with Academic Counselors here who guide you the right way. I know that languages in college move 10 times faster than high school and therefore I would not bomb the tests, but show what you know. Most people at USMA rise to the challenges whether or not they want to. All classes are hard here and maybe having that one extra assignment may not be that big of a deal. Bottom line, just show what you know and let them place you where you fit best.
As for summer and leadership opportunities, yes it is determined partially by your academics, but military and physical grades also factor in to this. Plus I think after one semester at USMA you will have almost everything figured out for studying. My first semester here has been rough but I think I've figured it out and now know how to precede forward. I should also point out that there are so many opportunities to do schools or travel, you can definitely find something, just have to apply.
Hope this helps!