1)
Depends on the caliber of the school you are looking at. For example, Milton (which is a really good school) has K-8 (day) and 9-12 (day and boarding). Rather than looking at the grades that a school has, I would look at the overall quality of the school and whether it is a good fit for your child.
2)
Both 6-12 and 1-12 schools are about the same. Normally these boarding schools with lower (below G9) schools would have a different management/school team for lower and upper schools. But the important thing about going to these schools is that you don’t need to transfer to another school in G9 or G10 (like the junior boarding schools).
3)
US boarding schools stress BOTH academics as well as extracurricular activities. High schools and colleges want students who are well rounded. It is important that the school will be able to cultivate in your child both excellence in academics as well as sports or arts.