Which branch is supposed to look beyond the interest of the separate states?

Answer 1

I would say the Judicial Branch (Federal Court and Supreme Court) as their decisions are supposed to be about the law and not about politics or geography. (in an ideal world, of course).

If not that, then the Executive Branch - the President and the Cabinet ae supposed to serve all of the U.S. and its protectorates (American Samoa, for example) and not be biased toward one state or another.

(Since I am Canadian, I may be way off base and not be addressing the question you have asked so if that is the case - I apologize)

Answer 2

See my answer linked to below that answers both your questions regarding the US Congress