You are inferring too much from Harvard. Nowhere do they say that European history, or any course, will “best prepare” you for Harvard; they say such course will provide a "strong base.: Anyway,as I said earlier, the Harvard recommendations are meant to cover most, but not all cases. Harvard says European history because most US HS history departments think that the history of the world centers around the US and Europe post-1450. Even the world history offering is disproportionately European based. Look at a typical HS offerings and find a Latin American History or Asian History or History of Ancient Greece and Rome. Good luck with that. 
I guarantee you that a US applicant that has a year of US history and 2 years of the history of another part of the world other than Europe will not be disadvantaged. But if you want to take AP European History, that’s fine, too. As I’ve told you many times, don’t take, or not take, a specific course just because a college “suggests” it.
My HS did not offer dual-enrollment, so all my classes were taken through the HS. My sciences were chem, astronomy, and 2 years of physics. FWIW, I took no classes during the summers.